$json = 'a:20:{s:3:"UID";s:32:"f9181c34c81e0f821708f3205b7b7d62";s:11:"X-EXTRAINFO";s:0:"";s:8:"LOCATION";s:0:"";s:11:"allDayEvent";s:2:"on";s:7:"CONTACT";s:0:"";s:11:"DESCRIPTION";s:0:"";s:12:"publish_down";s:8:"2020-8-1";s:10:"publish_up";s:8:"2020-8-1";s:13:"publish_down2";s:8:"2020-8-1";s:11:"publish_up2";s:8:"2020-8-1";s:7:"SUMMARY";s:6:"tesztt";s:3:"URL";s:0:"";s:11:"X-CREATEDBY";i:634;s:7:"DTSTART";i:1596232800;s:5:"DTEND";i:1596319199;s:5:"RRULE";a:4:{s:4:"FREQ";s:4:"none";s:5:"COUNT";i:1;s:8:"INTERVAL";s:1:"1";s:5:"BYDAY";s:24:"+1SA,+2SA,+3SA,+4SA,+5SA";}s:8:"MULTIDAY";s:1:"1";s:9:"NOENDTIME";s:1:"0";s:7:"X-COLOR";s:0:"";s:9:"LOCKEVENT";s:1:"0";}';


// csinálunk belöle egy tömböt
$data = unserialize($json);

// publish up kiíratás
print $data['publish_up'];

// átírjuk, ha kell a publish up értéket
$data['publish_up'] = '2015-05-05';

// visszalakítjuk
$data = serialize($data);

// kész :)
print $data;