You can perform a full factory reset from the serial console, at the logon prompt instead of entering your password type "RESET" (it needs to be in all uppercase and without the quote marks) at the resulting menu select either 1 or 2, as you have a XG85 I don't believe you will see option 3. (for larger appliances option 3 is useful as it will also erase the iView reporting database)

If you are unsure of how to set up the serial console connection please check out

Please be aware that the appliance will reboot and when it comes up it will need to be configured as expected, however you will not need to register the appliance again.