Upgrading to 3.71
m33-4
Refer to this guide if you are on official
firmware:
Unbricking your PSP
It will work on unbricked PSPs too.
You have a Phat with firmware lower than 3.52
m33-4
You need to at least be on 3.52 m33-3 or 3.52 m33-4 to upgrade to
3.71 m33-4. If you have a CFW that is not from the SE/OE/m33
series, recover to 1.50 first. I'm not sure if there will be any
conflicts during upgrading.
1) Download
3.52 m33. Extract all the files.
Readme file in folder:
PSP 3.52 M33 Installation
1 - If you are under official 1.50 PSP firmware, copy
kxploitpatcher and kxploitpatcher% folders from "1.50only/" of this
download to your "ms0:/PSP/GAME/ " and use it.
2 - Once done it, or if you are under any Special/Open Edition/M33,
copy M33CREATOR folder to your "ms0:/PSP/GAME " (1.50fw) or
"ms0:/PSP/GAME150 " (SE/OE)
3 - Download official 3.52 eboot, rename it to
352.PBP and place it on "ms0:/PSP/GAME/M33CREATOR/" or
"ms0:/PSP/GAME150/M33CREATOR/"
4 - Download official 1.50 eboot, rename it to
150.PBP and place it on "ms0:/PSP/GAME/M33CREATOR/" or
"ms0:/PSP/GAME150/M33CREATOR/"
5 - Go to Games/Memory Stick on your PSP, and execute "3.52 M33
CREATOR". Once finished, it will go back to the XMB.
6 - Go again to Games/Memory Stick on your PSP, and execute "3.52
M33UPDATE". You'll have to press X to agree to the
installation.
7 - When finished, press X to shut down and reboot manually.
8 - Enjoy your 3.52 M33 
2) Download 3.52 m33-4 update
and extract the files.
3) Copy the PSP folder to your memory stick, merging the
folders.
4) Run the update and follow instructions onscreen.
5) Congratulations being halfway there !
You have a Phat with 3.52 m33-3/4 or a Slim with a CFW
lower than 3.71 m33
1) Download 3.71 m33 and
extract.
Readme file in folder:
- Copy the UPDATE directory into /PSP/GAME/UPDATE and copy the
EBOOT.PBP of the 3.71 Sony updater
there but with name "371.PBP". Note: the program will not even show
if you copy it to a different folder than that.
- If you are in the fat psp, make sure you have configured the psp
to run the game folder in the 3.XX kernel.
- Execute the program, it requires 78% of battery. If you have
idstorage corrupted due to a downgrader, the program
will ask you to correct it automatically so the update can
continue.
- Press X, and the Sony updater will begin. Sit and enjoy while
Sony code installs 3.71 M33 
- When finished, the updater will ask you to reboot (pressing X or
O). DO IT. Do not skip this step
shutting down the psp by yourself, or you'll end with a
brick.
- You should be now in 3.71 M33
2) Download 3.71 m33-4 update and
extract.
3) Copy the UPDATE directory to /PSP/GAME/
4) If your PSP is a Phat, make sure the kernel is set to 3.71 in
the recovery menu.
5) Run the update, follow instructions onscreen.
6) Congratulations on 3.71 m33-4 !
Adding the 1.50
kernel to your Phat
THIS WILL NOT WORK ON THE SLIM, HOWEVER HARD YOU TRY.
This update adds the 1.50 kernel to your Phat, since the 3.71 m33
update does not do so due incompatibility with the slim. (Phat
users, curse the Slim 
)
You can decide not to do the addon, if you don't use 1.50 homebrew,
but a bugfix regarding returning from sleep mode is included in the
update. Not sure if it's in the 3.71 m33-4 update though. I
recommend doing this update, unless you need that precious flash0
space for your themes/experiments.
1) Download 1.50 kernel
addon v2 and extract.
2) Copy the eboot.pbp file to /PSP/GAME/UPDATE/.
3) Download the 1.50 eboot, name it 150.PBP
and place it in the root of your memory stick.
4) Make sure your kernel is set at 3.71 in the recovery menu.
5) Run the update.[/b]