How To Downgrade an unhacked PSP (Fat/Phat and Slim) with any Firmware revision including 3.51 and above.
Items you'll need before you start:
1. Unhacked PSP plugged via USB to your PC
2. A Sony Brand Pro Duo MS of 1gb or less (DO NOT USE NON-SONY
Brand MS for this step)
3. An Exacto Knife
4. A Charged Extra Battery that you can physically mod. This
battery will only function as a pandora battery after this
mod, you will not be able to use it to normally operate
Now, below is a very good guide with great pictures for hardmodding
a psp battery. These are however extremely complicated procedures
this guy is doing. I did it MUCH easier, no soldering. Use his
pix only to determine where to cut do not follow those instructions
unless your a soldering prodigy. I took my exacto knife, put the
tip of the blade under the correct leg of the chip and just popped
the leg off. Very simple procedure after you nut up and decide you
want to actually destroy your spare battery's normal operation
**** Please Note, there are a few different battery schematics.
If the pix below do not match your battery.. Google is your
friend. Also, my battery looked exactly like the one in these
pix, but the circuit board was not labeled at all, I didn't
have any printed C04 markings on my battery, but the layout is
the same, so I cut the leg that the guide below listed and it
put http: in front, apparently I can't post links yet ..
Congrats, you now have a Pandora Battery!! Now, lets make sure it
works. Make sure your PSP is not plugged into anything (USB or Power).
Put your pandora battery in your PSP. If the screen stays black and
the green power led comes on, you've successfully hardmod'd your
battery and are now in possession of a Pandora Battery.
Now to make a Magic Memory Stick.
1. Take out your pandora battery, set it aside for now.
2. Put your new Sony Pro Duo MS in your PSP
3. Plug your PSP into USB
4. Power on your PSP
5. On your PSP goto Settings and open USB Connection.
6. On your PC take note of what drive letter is assigned to ur PSP
7. Download http:
/magic_memory_stick.rar (make all that 1 line, I can't post links)
8. Unzip those files to c:\psptemp dir on your PC
9. copy msinst.exe msipl.bin and mspformat to c:\
10. Open CMD window on your PC
11. Change to c:\ by typeing cd\
12. type mspformat x (x = drive letter of PSP)
13. type msinst x msipl.bin (again x = drive letter of PSP)
14. Close the DOS CMD window
15. Copy the entire contents of c:\psptemp to the root of your PSP's MS
Congrats, you now have a Magic Memory Stick.
Now to downgrade your PSP.
1. Unplug your PSP from all power and USB.
2. Your PSP should be completely off.
3. Put your Magic Memory Stick in.
4. Put your Pandora Battery in.
5. You should see your PSP boot to a menu with O /_\ X  options
6. Select the option that says Install 3.71 M33 (I'm doing this
from Memory and thinks its menu button X)
7. Your PSP should now be flashing, DO NOT TOUCH YOUR PSP while
this process is running. Only push buttons if prompted to.
8. Once your PSP is done flashing remove your Pandora battery
put in a normal battery or AC but leave in your Magic Memory
9. Boot your newly mod'd PSP. On your PSP goto Game, then Memory
Stick, select the 3.71M 33-3 option and it will flash your PSP
Congrats, you now have a homebrew enabled PSP running the lastest
and greatest firmware as of today (Dec 1, 2007) 3.71M33-3.