This is how you downgrade and Official Firmware (OFW) higher than 6.20 back down to 6.20 OFW on your PSP. This again is able to be used on OFW PSP's and also use this at your own risk.
Take the file after downloading and extract the folder 6.XX Downgrader and place it in the ms0:/PSP/GAME folder and run it from the XMB and hit X to start the downgrade.
NOTE: USE AT YOUR OWN RISK
RemoteJoyLite Video tutorial: http://www.youtube.com/watch?v=VwJMuFmTjgk