How to Downgrade 6.20 TN-B

Downgrading your psp is not really recommended, since every time you update or upgrade your psp, there is a chance of bricking your psp. especially 3001 and Go models. I do not recommend doing it, but here is the "How to" anyways:

1.   Start 6.20 TN-B
2. Download the following files:
   • PSARDumper
   • OFW (EBOOT.PBP) Update
3. Place PSARDumper in the PSP/GAME folder like you would any other homebrew and copy the OFW EBOOT.PBP to the root of your memory stick
4. Start PSARDumper and press circle
5. After the program finishes, connect the USB cable to your PSP and navigate to the folder F0, which should now be at the root of your memory stick
6. Copy /F0/vsh/etc/version.txt to /seplugins/version.txt
7. Go back to your PSP XMB and press select to bring up the VSH menu. Go to “Fake Index.dat” and change it to “enabled”. Next, select “restart vsh”
8. Copy the OFW EBOOT.PBP to “PSP/GAME/UPDATE/EBOOT.PBP”
9. On your PSP start the OFW installation as you would any other firmware update
10. After it finishes, the PSP will restart and a blue screen will appear saying that the PSP settings are invalid. Press circle and the PSP will restore default settings and restart. You now have OFW installed.

Source: psp-hacks