This package downgrades your current PSP firmware to 6.39. You can use an other firmware to downgrade to if you like, but you should download it elsewhere. I have tested the Downgrader with the 6.60 firmware but that was a no-go :(

Copy the contents from this package (/UPDATE and /Downgrade) to your PSP in the directory /PSP/GAME. Then start your PSP and run Downgrade from youw XMB. Remember that your battery should be charged OR the PSP should be cord-powered