button.titleLabel.font = [UIFont systemFontOfSize:size];

[myButton.titleLabel setFont:[UIFont fontWithName:@"Helvetica-Bold" size:13.0]];

NSArray *familyNames = [[NSArray alloc] initWithArray:[UIFont familyNames]];
NSArray *fontNames;
NSInteger indFamily, indFont;
for (indFamily=0; indFamily<[familyNames count]; ++indFamily)
{
    NSLog(@"Family name: %@", [familyNames objectAtIndex:indFamily]);
    fontNames = [[NSArray alloc] initWithArray:
            [UIFont fontNamesForFamilyName:
            [familyNames objectAtIndex:indFamily]]];
    for (indFont=0; indFont<[fontNames count]; ++indFont)
    {
        NSLog(@"    Font name: %@", [fontNames objectAtIndex:indFont]);
    }
}

2012-04-02 11:36:34.395 MyApp[3579:707] Family name: Thonburi
2012-04-02 11:36:34.398 MyApp[3579:707]     Font name: Thonburi-Bold
2012-04-02 11:36:34.402 MyApp[3579:707]     Font name: Thonburi
2012-04-02 11:36:34.405 MyApp[3579:707] Family name: Snell Roundhand
2012-04-02 11:36:34.408 MyApp[3579:707]     Font name: SnellRoundhand-Bold
2012-04-02 11:36:34.411 MyApp[3579:707]     Font name: SnellRoundhand-Black
2012-04-02 11:36:34.415 MyApp[3579:707]     Font name: SnellRoundhand
2012-04-02 11:36:34.418 MyApp[3579:707] Family name: Academy Engraved LET
2012-04-02 11:36:34.421 MyApp[3579:707]     Font name: AcademyEngravedLetPlain
2012-04-02 11:36:34.424 MyApp[3579:707] Family name: Marker Felt
2012-04-02 11:36:34.427 MyApp[3579:707]     Font name: MarkerFelt-Wide
2012-04-02 11:36:34.430 MyApp[3579:707]     Font name: MarkerFelt-Thin
2012-04-02 11:36:34.434 MyApp[3579:707] Family name: Geeza Pro
2012-04-02 11:36:34.437 MyApp[3579:707]     Font name: GeezaPro-Bold
2012-04-02 11:36:34.441 MyApp[3579:707]     Font name: GeezaPro
2012-04-02 11:36:34.445 MyApp[3579:707] Family name: Arial Rounded MT Bold
2012-04-02 11:36:34.448 MyApp[3579:707]     Font name: ArialRoundedMTBold
2012-04-02 11:36:34.451 MyApp[3579:707] Family name: Trebuchet MS
2012-04-02 11:36:34.455 MyApp[3579:707]     Font name: TrebuchetMS
2012-04-02 11:36:34.458 MyApp[3579:707]     Font name: TrebuchetMS-Bold
2012-04-02 11:36:34.461 MyApp[3579:707]     Font name: TrebuchetMS-Italic
2012-04-02 11:36:34.464 MyApp[3579:707]     Font name: Trebuchet-BoldItalic
2012-04-02 11:36:34.467 MyApp[3579:707] Family name: Arial
2012-04-02 11:36:34.471 MyApp[3579:707]     Font name: Arial-BoldMT
2012-04-02 11:36:34.474 MyApp[3579:707]     Font name: ArialMT
2012-04-02 11:36:34.477 MyApp[3579:707]     Font name: Arial-ItalicMT
2012-04-02 11:36:34.480 MyApp[3579:707]     Font name: Arial-BoldItalicMT
2012-04-02 11:36:34.483 MyApp[3579:707] Family name: Marion
2012-04-02 11:36:34.487 MyApp[3579:707]     Font name: Marion-Regular
2012-04-02 11:36:34.491 MyApp[3579:707]     Font name: Marion-Bold
2012-04-02 11:36:34.494 MyApp[3579:707]     Font name: Marion-Italic
2012-04-02 11:36:34.503 MyApp[3579:707] Family name: Gurmukhi MN
2012-04-02 11:36:34.507 MyApp[3579:707]     Font name: GurmukhiMN
2012-04-02 11:36:34.511 MyApp[3579:707]     Font name: GurmukhiMN-Bold
2012-04-02 11:36:34.514 MyApp[3579:707] Family name: Malayalam Sangam MN
2012-04-02 11:36:34.518 MyApp[3579:707]     Font name: MalayalamSangamMN-Bold
2012-04-02 11:36:34.522 MyApp[3579:707]     Font name: MalayalamSangamMN
2012-04-02 11:36:34.525 MyApp[3579:707] Family name: Bradley Hand
2012-04-02 11:36:34.529 MyApp[3579:707]     Font name: BradleyHandITCTT-Bold
2012-04-02 11:36:34.532 MyApp[3579:707] Family name: Kannada Sangam MN
2012-04-02 11:36:34.536 MyApp[3579:707]     Font name: KannadaSangamMN
2012-04-02 11:36:34.540 MyApp[3579:707]     Font name: KannadaSangamMN-Bold
2012-04-02 11:36:34.544 MyApp[3579:707] Family name: Bodoni 72 Oldstyle
2012-04-02 11:36:34.548 MyApp[3579:707]     Font name: BodoniSvtyTwoOSITCTT-Book
2012-04-02 11:36:34.552 MyApp[3579:707]     Font name: BodoniSvtyTwoOSITCTT-Bold
2012-04-02 11:36:34.555 MyApp[3579:707]     Font name: BodoniSvtyTwoOSITCTT-BookIt
2012-04-02 11:36:34.559 MyApp[3579:707] Family name: Cochin
2012-04-02 11:36:34.562 MyApp[3579:707]     Font name: Cochin
2012-04-02 11:36:34.566 MyApp[3579:707]     Font name: Cochin-BoldItalic
2012-04-02 11:36:34.570 MyApp[3579:707]     Font name: Cochin-Italic
2012-04-02 11:36:34.573 MyApp[3579:707]     Font name: Cochin-Bold
2012-04-02 11:36:34.577 MyApp[3579:707] Family name: Sinhala Sangam MN
2012-04-02 11:36:34.581 MyApp[3579:707]     Font name: SinhalaSangamMN
2012-04-02 11:36:34.584 MyApp[3579:707]     Font name: SinhalaSangamMN-Bold
2012-04-02 11:36:34.588 MyApp[3579:707] Family name: Hiragino Kaku Gothic ProN
2012-04-02 11:36:34.592 MyApp[3579:707]     Font name: HiraKakuProN-W6
2012-04-02 11:36:34.596 MyApp[3579:707]     Font name: HiraKakuProN-W3
2012-04-02 11:36:34.599 MyApp[3579:707] Family name: Papyrus
2012-04-02 11:36:34.603 MyApp[3579:707]     Font name: Papyrus-Condensed
2012-04-02 11:36:34.607 MyApp[3579:707]     Font name: Papyrus
2012-04-02 11:36:34.614 MyApp[3579:707] Family name: Verdana
2012-04-02 11:36:34.620 MyApp[3579:707]     Font name: Verdana
2012-04-02 11:36:34.626 MyApp[3579:707]     Font name: Verdana-Bold
2012-04-02 11:36:34.674 MyApp[3579:707]     Font name: Verdana-BoldItalic
2012-04-02 11:36:34.690 MyApp[3579:707]     Font name: Verdana-Italic
2012-04-02 11:36:34.730 MyApp[3579:707] Family name: Zapf Dingbats
2012-04-02 11:36:34.748 MyApp[3579:707]     Font name: ZapfDingbatsITC
2012-04-02 11:36:34.752 MyApp[3579:707] Family name: Courier
2012-04-02 11:36:34.757 MyApp[3579:707]     Font name: Courier-Bold
2012-04-02 11:36:34.762 MyApp[3579:707]     Font name: Courier
2012-04-02 11:36:34.769 MyApp[3579:707]     Font name: Courier-BoldOblique
2012-04-02 11:36:34.778 MyApp[3579:707]     Font name: Courier-Oblique
2012-04-02 11:36:34.786 MyApp[3579:707] Family name: Hoefler Text
2012-04-02 11:36:34.793 MyApp[3579:707]     Font name: HoeflerText-Black
2012-04-02 11:36:34.802 MyApp[3579:707]     Font name: HoeflerText-Italic
2012-04-02 11:36:34.810 MyApp[3579:707]     Font name: HoeflerText-Regular
2012-04-02 11:36:34.819 MyApp[3579:707]     Font name: HoeflerText-BlackItalic
2012-04-02 11:36:34.827 MyApp[3579:707] Family name: Euphemia UCAS
2012-04-02 11:36:34.836 MyApp[3579:707]     Font name: EuphemiaUCAS-Bold
2012-04-02 11:36:34.843 MyApp[3579:707]     Font name: EuphemiaUCAS
2012-04-02 11:36:34.848 MyApp[3579:707]     Font name: EuphemiaUCAS-Italic
2012-04-02 11:36:34.853 MyApp[3579:707] Family name: Helvetica
2012-04-02 11:36:34.857 MyApp[3579:707]     Font name: Helvetica-LightOblique
2012-04-02 11:36:34.863 MyApp[3579:707]     Font name: Helvetica
2012-04-02 11:36:34.873 MyApp[3579:707]     Font name: Helvetica-Oblique
2012-04-02 11:36:34.876 MyApp[3579:707]     Font name: Helvetica-BoldOblique
2012-04-02 11:36:34.880 MyApp[3579:707]     Font name: Helvetica-Bold
2012-04-02 11:36:34.884 MyApp[3579:707]     Font name: Helvetica-Light
2012-04-02 11:36:34.887 MyApp[3579:707] Family name: Hiragino Mincho ProN
2012-04-02 11:36:34.892 MyApp[3579:707]     Font name: HiraMinProN-W3
2012-04-02 11:36:34.898 MyApp[3579:707]     Font name: HiraMinProN-W6
2012-04-02 11:36:34.902 MyApp[3579:707] Family name: Bodoni Ornaments
2012-04-02 11:36:34.905 MyApp[3579:707]     Font name: BodoniOrnamentsITCTT
2012-04-02 11:36:34.923 MyApp[3579:707] Family name: Apple Color Emoji
2012-04-02 11:36:34.938 MyApp[3579:707]     Font name: AppleColorEmoji
2012-04-02 11:36:34.942 MyApp[3579:707] Family name: Optima
2012-04-02 11:36:34.946 MyApp[3579:707]     Font name: Optima-ExtraBlack
2012-04-02 11:36:34.950 MyApp[3579:707]     Font name: Optima-Italic
2012-04-02 11:36:34.954 MyApp[3579:707]     Font name: Optima-Regular
2012-04-02 11:36:34.965 MyApp[3579:707]     Font name: Optima-BoldItalic
2012-04-02 11:36:34.969 MyApp[3579:707]     Font name: Optima-Bold
2012-04-02 11:36:34.972 MyApp[3579:707] Family name: Gujarati Sangam MN
2012-04-02 11:36:34.985 MyApp[3579:707]     Font name: GujaratiSangamMN
2012-04-02 11:36:34.989 MyApp[3579:707]     Font name: GujaratiSangamMN-Bold
2012-04-02 11:36:34.993 MyApp[3579:707] Family name: Devanagari Sangam MN
2012-04-02 11:36:34.998 MyApp[3579:707]     Font name: DevanagariSangamMN
2012-04-02 11:36:35.002 MyApp[3579:707]     Font name: DevanagariSangamMN-Bold
2012-04-02 11:36:35.006 MyApp[3579:707] Family name: Times New Roman
2012-04-02 11:36:35.017 MyApp[3579:707]     Font name: TimesNewRomanPS-ItalicMT
2012-04-02 11:36:35.021 MyApp[3579:707]     Font name: TimesNewRomanPS-BoldMT
2012-04-02 11:36:35.032 MyApp[3579:707]     Font name: TimesNewRomanPSMT
2012-04-02 11:36:35.037 MyApp[3579:707]     Font name: TimesNewRomanPS-BoldItalicMT
2012-04-02 11:36:35.041 MyApp[3579:707] Family name: Kailasa
2012-04-02 11:36:35.045 MyApp[3579:707]     Font name: Kailasa
2012-04-02 11:36:35.050 MyApp[3579:707]     Font name: Kailasa-Bold
2012-04-02 11:36:35.053 MyApp[3579:707] Family name: Telugu Sangam MN
2012-04-02 11:36:35.064 MyApp[3579:707]     Font name: TeluguSangamMN-Bold
2012-04-02 11:36:35.068 MyApp[3579:707]     Font name: TeluguSangamMN
2012-04-02 11:36:35.071 MyApp[3579:707] Family name: Heiti SC
2012-04-02 11:36:35.099 MyApp[3579:707]     Font name: STHeitiSC-Medium
2012-04-02 11:36:35.107 MyApp[3579:707]     Font name: STHeitiSC-Light
2012-04-02 11:36:35.111 MyApp[3579:707] Family name: Futura
2012-04-02 11:36:35.115 MyApp[3579:707]     Font name: Futura-Medium
2012-04-02 11:36:35.119 MyApp[3579:707]     Font name: Futura-CondensedExtraBold
2012-04-02 11:36:35.122 MyApp[3579:707]     Font name: Futura-CondensedMedium
2012-04-02 11:36:35.135 MyApp[3579:707]     Font name: Futura-MediumItalic
2012-04-02 11:36:35.155 MyApp[3579:707] Family name: Bodoni 72
2012-04-02 11:36:35.160 MyApp[3579:707]     Font name: BodoniSvtyTwoITCTT-BookIta
2012-04-02 11:36:35.164 MyApp[3579:707]     Font name: BodoniSvtyTwoITCTT-Book
2012-04-02 11:36:35.168 MyApp[3579:707]     Font name: BodoniSvtyTwoITCTT-Bold
2012-04-02 11:36:35.171 MyApp[3579:707] Family name: Baskerville
2012-04-02 11:36:35.183 MyApp[3579:707]     Font name: Baskerville-SemiBoldItalic
2012-04-02 11:36:35.187 MyApp[3579:707]     Font name: Baskerville-Bold
2012-04-02 11:36:35.197 MyApp[3579:707]     Font name: Baskerville-Italic
2012-04-02 11:36:35.245 MyApp[3579:707]     Font name: Baskerville-BoldItalic
2012-04-02 11:36:35.253 MyApp[3579:707]     Font name: Baskerville-SemiBold
2012-04-02 11:36:35.258 MyApp[3579:707]     Font name: Baskerville
2012-04-02 11:36:35.262 MyApp[3579:707] Family name: Chalkboard SE
2012-04-02 11:36:35.266 MyApp[3579:707]     Font name: ChalkboardSE-Regular
2012-04-02 11:36:35.269 MyApp[3579:707]     Font name: ChalkboardSE-Bold
2012-04-02 11:36:35.279 MyApp[3579:707]     Font name: ChalkboardSE-Light
2012-04-02 11:36:35.284 MyApp[3579:707] Family name: Heiti TC
2012-04-02 11:36:35.288 MyApp[3579:707]     Font name: STHeitiTC-Medium
2012-04-02 11:36:35.299 MyApp[3579:707]     Font name: STHeitiTC-Light
2012-04-02 11:36:35.305 MyApp[3579:707] Family name: Copperplate
2012-04-02 11:36:35.310 MyApp[3579:707]     Font name: Copperplate
2012-04-02 11:36:35.313 MyApp[3579:707]     Font name: Copperplate-Light
2012-04-02 11:36:35.317 MyApp[3579:707]     Font name: Copperplate-Bold
2012-04-02 11:36:35.320 MyApp[3579:707] Family name: Party LET
2012-04-02 11:36:35.334 MyApp[3579:707]     Font name: PartyLetPlain
2012-04-02 11:36:35.338 MyApp[3579:707] Family name: American Typewriter
2012-04-02 11:36:35.351 MyApp[3579:707]     Font name: AmericanTypewriter-CondensedLight
2012-04-02 11:36:35.357 MyApp[3579:707]     Font name: AmericanTypewriter-Light
2012-04-02 11:36:35.361 MyApp[3579:707]     Font name: AmericanTypewriter-Bold
2012-04-02 11:36:35.364 MyApp[3579:707]     Font name: AmericanTypewriter
2012-04-02 11:36:35.368 MyApp[3579:707]     Font name: AmericanTypewriter-CondensedBold
2012-04-02 11:36:35.372 MyApp[3579:707]     Font name: AmericanTypewriter-Condensed
2012-04-02 11:36:35.384 MyApp[3579:707] Family name: AppleGothic
2012-04-02 11:36:35.400 MyApp[3579:707]     Font name: AppleGothic
2012-04-02 11:36:35.406 MyApp[3579:707] Family name: Bangla Sangam MN
2012-04-02 11:36:35.411 MyApp[3579:707]     Font name: BanglaSangamMN-Bold
2012-04-02 11:36:35.414 MyApp[3579:707]     Font name: BanglaSangamMN
2012-04-02 11:36:35.418 MyApp[3579:707] Family name: Noteworthy
2012-04-02 11:36:35.422 MyApp[3579:707]     Font name: Noteworthy-Light
2012-04-02 11:36:35.432 MyApp[3579:707]     Font name: Noteworthy-Bold
2012-04-02 11:36:35.436 MyApp[3579:707] Family name: Zapfino
2012-04-02 11:36:35.443 MyApp[3579:707]     Font name: Zapfino
2012-04-02 11:36:35.448 MyApp[3579:707] Family name: Tamil Sangam MN
2012-04-02 11:36:35.452 MyApp[3579:707]     Font name: TamilSangamMN
2012-04-02 11:36:35.456 MyApp[3579:707]     Font name: TamilSangamMN-Bold
2012-04-02 11:36:35.459 MyApp[3579:707] Family name: DB LCD Temp
2012-04-02 11:36:35.463 MyApp[3579:707]     Font name: DBLCDTempBlack
2012-04-02 11:36:35.467 MyApp[3579:707] Family name: Arial Hebrew
2012-04-02 11:36:35.471 MyApp[3579:707]     Font name: ArialHebrew
2012-04-02 11:36:35.475 MyApp[3579:707]     Font name: ArialHebrew-Bold
2012-04-02 11:36:35.479 MyApp[3579:707] Family name: Chalkduster
2012-04-02 11:36:35.482 MyApp[3579:707]     Font name: Chalkduster
2012-04-02 11:36:35.486 MyApp[3579:707] Family name: Georgia
2012-04-02 11:36:35.490 MyApp[3579:707]     Font name: Georgia-Italic
2012-04-02 11:36:35.493 MyApp[3579:707]     Font name: Georgia-BoldItalic
2012-04-02 11:36:35.497 MyApp[3579:707]     Font name: Georgia-Bold
2012-04-02 11:36:35.501 MyApp[3579:707]     Font name: Georgia
2012-04-02 11:36:35.504 MyApp[3579:707] Family name: Helvetica Neue
2012-04-02 11:36:35.508 MyApp[3579:707]     Font name: HelveticaNeue-Bold
2012-04-02 11:36:35.511 MyApp[3579:707]     Font name: HelveticaNeue-CondensedBlack
2012-04-02 11:36:35.515 MyApp[3579:707]     Font name: HelveticaNeue-Medium
2012-04-02 11:36:35.518 MyApp[3579:707]     Font name: HelveticaNeue
2012-04-02 11:36:35.522 MyApp[3579:707]     Font name: HelveticaNeue-Light
2012-04-02 11:36:35.526 MyApp[3579:707]     Font name: HelveticaNeue-CondensedBold
2012-04-02 11:36:35.529 MyApp[3579:707]     Font name: HelveticaNeue-LightItalic
2012-04-02 11:36:35.532 MyApp[3579:707]     Font name: HelveticaNeue-UltraLightItalic
2012-04-02 11:36:35.536 MyApp[3579:707]     Font name: HelveticaNeue-UltraLight
2012-04-02 11:36:35.540 MyApp[3579:707]     Font name: HelveticaNeue-BoldItalic
2012-04-02 11:36:35.543 MyApp[3579:707]     Font name: HelveticaNeue-Italic
2012-04-02 11:36:35.547 MyApp[3579:707] Family name: Gill Sans
2012-04-02 11:36:35.551 MyApp[3579:707]     Font name: GillSans-LightItalic
2012-04-02 11:36:35.555 MyApp[3579:707]     Font name: GillSans-BoldItalic
2012-04-02 11:36:35.558 MyApp[3579:707]     Font name: GillSans-Italic
2012-04-02 11:36:35.562 MyApp[3579:707]     Font name: GillSans
2012-04-02 11:36:35.565 MyApp[3579:707]     Font name: GillSans-Bold
2012-04-02 11:36:35.569 MyApp[3579:707]     Font name: GillSans-Light
2012-04-02 11:36:35.572 MyApp[3579:707] Family name: Palatino
2012-04-02 11:36:35.576 MyApp[3579:707]     Font name: Palatino-Roman
2012-04-02 11:36:35.580 MyApp[3579:707]     Font name: Palatino-Bold
2012-04-02 11:36:35.583 MyApp[3579:707]     Font name: Palatino-BoldItalic
2012-04-02 11:36:35.587 MyApp[3579:707]     Font name: Palatino-Italic
2012-04-02 11:36:35.591 MyApp[3579:707] Family name: Courier New
2012-04-02 11:36:35.594 MyApp[3579:707]     Font name: CourierNewPSMT
2012-04-02 11:36:35.598 MyApp[3579:707]     Font name: CourierNewPS-BoldMT
2012-04-02 11:36:35.601 MyApp[3579:707]     Font name: CourierNewPS-BoldItalicMT
2012-04-02 11:36:35.605 MyApp[3579:707]     Font name: CourierNewPS-ItalicMT
2012-04-02 11:36:35.608 MyApp[3579:707] Family name: Oriya Sangam MN
2012-04-02 11:36:35.612 MyApp[3579:707]     Font name: OriyaSangamMN-Bold
2012-04-02 11:36:35.616 MyApp[3579:707]     Font name: OriyaSangamMN
2012-04-02 11:36:35.619 MyApp[3579:707] Family name: Didot
2012-04-02 11:36:35.623 MyApp[3579:707]     Font name: Didot-Italic
2012-04-02 11:36:35.627 MyApp[3579:707]     Font name: Didot
2012-04-02 11:36:35.630 MyApp[3579:707]     Font name: Didot-Bold
2012-04-02 11:36:35.634 MyApp[3579:707] Family name: Bodoni 72 Smallcaps
2012-04-02 11:36:35.638 MyApp[3579:707]     Font name: BodoniSvtyTwoSCITCTT-Book

[button.titleLabel setFont: [button.titleLabel.font fontWithSize: sizeYouWant]];

button.titleLabel?.font = button.titleLabel?.font.fontWithSize(sizeYouWant)

button.titleLabel.font = [UIFont fontWithName:@"YOUR FONTNAME" size:12.0f]

button.titleLabel.font = [UIFont systemFontOfSize:14.0];

[_button.titleLabel setFont:[UIFont systemFontOfSize:15]];

shareButton.titleLabel?.font = UIFont.systemFontOfSize(size)

[LoginButton.titleLabel setFont:[UIFont boldSystemFontOfSize:15.0f*Ratio]];

button.titleLabel.font = <whatever font you want>

button.titleLabel.text = @"something";

[button setTitle:@"My title" forState:UIControlStateNormal]; //or whatever you want the control state to be

[btn_submit.titleLabel setFont:[UIFont systemFontOfSize:14.0f]];

- (void)awakeFromNib{

    [super awakeFromNib];

    [self setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [self.titleLabel setFont:[UIFont fontWithName:@"font"
                                     size:self.titleLabel.font.pointSize]];
    [self setContentHorizontalAlignment:UIControlContentHorizontalAlignmentCenter];
}

myButton.titleLabel?.font = myButton.titleLabel?.font.withSize(40)