This a very simple app, just put the official 6.20 updater in (ms0/ef0 accordingly)PSP/GAME/UPDATE, and run the downgrader, this will reboot into the 6.20 "update", and allow you to downgrade to 6.20.
YOU can get OFW 6.20(also known as 6.20 updater) and 6.20 pro B6 HERE