You have to open your VSH menu and select FLASH1 as your USB Device.
Then browse through the folders (I forgot which one, sorry)
You will then find version.txt
It's plaintext so you can edit it, if it's not open it with a hex editor or something and find your version #.
Once you find it, replace it with 6.66 or something. anything higher than what is available.
Sony doesn't create checks for SUPER HIGH FW VERSIONS
Then save it.
Disconnect USB and go to your RECOVERY MENU.
Go to "Configuration" and select Use version.txt
Hit (x) to enable it. Restart your PSP.
hopefully this will work.