You don't need to unlock bootloader or root or do anything special to downgrade. You just need to check which format is compatible for your device : UL-ASUS_I006D-ASUS or UL-I006D-ASUS. The file name has to be EXACT for the update for upgrade or downgrade to TRIGGER. It's a bit of a trial and error but rest assured it works once you put the file and it's proper naming convention in the root directory. Best to reboot if it doesn't recognize after 2min. Hope that helps.