The following revert example is based on Web Gui revert method. Please ensure you get the correct web revert file for right device, failure which you may brick the device!!
Hard re-set the device, then login default again.
Go to "Administration --> Firmware Upgrade" page, browse to the correct file, click "Upgrade" to proceed.
DO NOT touch anything until it's fully competed!!
Make sure it's fully completed (in few minutes), if there is no default screen show up, enter the default factory IP of the device, eg, 192.168.1.1
Enter the default factory admin ID & password
Back to factory firmware!
Wait, don't do anything first, hard reset the device again, after that you may do anything you want!