Asus Zenfone 4,5&6 are now able to upgrade their old android to newest Android 4.4 !.To update, go to Settings> About> System Update>Check for update.
The update will be available by batches. So if you have not receive update notification on your ZenFone 5 or ZenFone 6, do check again later and you shall receive within the short time.
As previously announced, Android 4.4 Kit Kat update is also available for ZenFone 4.
To download, please log on;
ZenFone 4 - http://docdroid.net/iogg
ZenFone 5 & 6 - http://docdroid.net/iogh
