The RESET button is located at the rear side of your firewall’s housing. The following image shows as an example for the F12 the RESET button on the rear side:

Before You Begin
Ensure that the power supply of your firewall will not be interrupted unless you want to render your firewall non-functional!
In case you were running a firmware version with hotfixes installed on top of the firmware, you must install them manually after restoring the ART recovery backup!
To Restore an ART Recovery Backup, perform the Following Steps
The RESET button is located inside of the housing, so you must use a ballpoint pen or another appropriate pointed object to touch the button.
To perform a restore of an ART recovery backup, position the pointed object in the hole.
To ensure that you exceed the given time limit of 10 seconds, press the point object at last for 11 seconds. Then release the pointed object.
The firewall will immediately begin restoring the ISO file in the /ART directory and restore the PAR backup.
Remember to install all relevant hotfixes after restoring the ART recovery backup and the PAR backup.