Download and copy the firmware zip file to your internal storage, and make sure the filename is UL-Z00A-WW-4.21.40.352-user.zip.
Make sure to move all important apps back to internal storage, as there is no APP2SD support on Android 6.0 Marshmallow. Also, please make sure you have a backup of your files ready, just in case.
Restart your device.
A“System Update File Detected” notification should appear. Tap the notification and select the correct update package.
Make sure your battery is above 60%
If you’re sure that you are ready, tap the OK button.
The device should reboot to recovery mode and start updating your device. This will take a while. DO NOT REBOOT YOUR PHONE during this part.
Once the update is done, the device should restart by itself. Optimization of apps will take time depending on how many apps are installed on your phone.
I can't able to post link to the firmware but you can get on
Asus website/Asus Zenfone 2 (Ze551ml)/Support/AndroidOS/Firmwares.