Step-1: Download Android 4.4 KitKat firmware versions 5.3.3.3 and 5.2.2.2 for your ZenFone 5
Download links:
UL-ASUS_T00F-WW-5.3.3.3-user.zip
UL-ASUS_T00F-WW-5.2.2.2-user.zip
Note: Do not extract the downloaded links.
Step-2: Connect the ZenFone to the PC and copy the version 5.3.3.3 only to the phone's internal storage. Do not copy the v5.2.2.2
Step-3: Disable the USB cable. Your phone will show a notification, 'System Update File Detected. Touch to start the system update'
Step-4: Touch the notification to get a pop-up, which will show the file name of the KitKat firmware v5.3.3.3. Select it and tap OK
Step-5: The phone will now enter recovery mode and install the aforementioned KitKat firmware. After the installation, the phone will restart automatically
Step-6: Now flash KitKat firmware v5.2.2.2 on your phone
Step-7: Connect the phone with the PC and move the KitKat file to the internal storage
Step-8: Like the previous KitKat firmware, you should get a notification about the update. Simply touch it to get a pop-up and tap OK to install 5.2.2.2. This should downgrade your software version KitKat completely
Step-9: A reboot is required after the process
After the process go to Settings>> About>> Software information>> Build number, and it should show 2.21.40.44.