Be warned: You can mess up the calibration of your detector very easily by doing this. This will take you to a hidden menu on both the V3, and the V3i, but my understanding is this should only be done on the V3i. Who knows for sure, Whites is very secretive these days.
To do a master reset... (Factory Reset)
1. Press ON/OFF, and hold. Do not release.
2. Press MENU/TAB, and hold.
3. RELEASE ON/OFF
4. Push ON/OFF again.
From here you will see the master reset option.