//code to setup AVMutableComposition
...
//code to setup CATextLayer and AVMutableVideoComposition
    CALayer *animatedTitleLayer = [CALayer layer];
    CATextLayer *titleLayer = [[CATextLayer alloc] init];
    titleLayer.string = @"Text I want to change at an interval";
    titleLayer.alignmentMode = kCAAlignmentCenter;
    titleLayer.bounds = CGRectMake(150, 50, 124, 354);
    titleLayer.position = CGPointMake(120, 270);
    titleLayer.bounds = CGRectIntegral(CGRectMake(0, 0, 250, 150));
    titleLayer.opacity = 1;
    titleLayer.backgroundColor = [UIColor purpleColor].CGColor;
    [animatedTitleLayer addSublayer:titleLayer];
    animatedTitleLayer.position = CGPointMake(40, 5);
    CALayer *parentLayer = [CALayer layer];
    CALayer *videoLayer = [CALayer layer];
    parentLayer.frame = CGRectMake(0, 0, 320, 480);
    videoLayer.frame = CGRectMake(0, 0, 320, 480);
    [parentLayer addSublayer:videoLayer];
    [parentLayer addSublayer:animatedTitleLayer];
    parentLayer.preferredTransform = rotationTransform;
    AVMutableVideoComposition *videoComposition;
    videoComposition = [AVMutableVideoComposition videoComposition];
    videoComposition.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

//code to setup AVExportSession
...

titleLayer.name = "changingTextLayer"

for (CALayer *layer in [superLayerOfMyLayer sublayers]) {
        if ([[layer name] isEqualToString:"changingTextLayer"]) {
            return layer;
        }
    }

titleLayer.string = @"Changed text";