Download KitKat Beta for Zenfone 5 and Zenfone 6

DWYOR (Do it With Your Own Risk) 
Want stable version? Wait for the Official/Final then. Because this version was launched on 17-07-2014 (Beta version).
Links from ASUS OTA update center.

Zenfone 5 WW Version A500CG/A501CG

Zenfone 5 TW Version A500CG/A501CG

Zenfone 6 WW Version A600CG

Zenfone 6 TW Version A600CG

How to Flash?
Flash WW Firmware v 1.15.40.35 then flash this zip via ADB/Manual update