You can grab the PurpleAir Utility app The PurpleAir Utility and once you’re connected, there’s an action in the ‘Select a Flash Action’ menu for Erase ALL Memory (select that, then click Flash). You’ll want to grab a copy of the current firmware first, so you can follow-up the erase step with flashing the correct firmware (same menu as the erase, Choose a firmware file, then Flash).
At that point, you should be fully reset, if that’s what you’re wanting to try.