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If you want to upgrade your PSP to v3.90 M33, here is your guide.

In order to complete the guide, you will need this file

This proceedure will NOT downgrade a PSP, you need to be running a custom firmware or 1.50 firmware PSP to begin with.
First of all, charge battery to 100% to avoid any nags/failures and keep the power cord connected at all times during
the proceedure. It is also advisable that you remove any UMDs from the drive before beginning.

Note: X: represents the drive letter of your PSP on your computer.
Also needed: Firmwares 1.50, 3.52 and 3.90 from here.

Step 1
If you have a PSP running custom firmware 3.52 M33 or higher, skip this step.

1. Copy the M33CREATOR folder from the 3.52_m33.zip into X:\PSP\GAME
2. Copy the 352.PBP and 150.PBP from the PSP folder in this archive to X:\PSP\GAME\M33CREATOR
3. Disconnect the PSP and run the M33CREATOR application under Game->Memory stick from the XMB

Follow the instructions on screen and your PSP will restart with 3.52 M33, you should now delete the M33CREATOR, M33INSTALLER folders from your PSP in X:\PSP\GAME.


Step 2
If you have a PSP running custom firmware 3.52 M33-3 or higher, skip this step.

1. Copy the PSP folder from the 3.52_m33update2.zip into X:\, you don't need any other files yet.
2. Disconnect the PSP and run the M33 update application under Game->Memory stick from the XMB.
3. Follow the on-screen instructions to perform the update.
4. When returned to the XMB, reconnect the PSP and remove the M33Update folder.
5. Repeat this process for 3.52_m33update3.zip to update the PSP to 3.52 M33-3.


Step 3
1. Copy the UPDATE folder from 390m33.rar into X:\PSP\GAME
2. Copy the 390.PBP from PSP in this archive to X:\PSP\GAME\UPDATE
3. Disconnect the PSP and run the "PSP Update ver. 3.90".
4. Follow the on-screen instructions, you'll be presented with the official Sony updater.
5. Don't panic, this is normal, install the update and you'll be at 3.90 M33 when the system reboots.

You can use the official Network update feature to retrieve any additional updates, such as 3.90 M33-2 etc.

Step 4
1.50 kernel addon - this section is optional, will only work with the old(fat/phat) PSP and is not needed unless you wish to run older homebrew applications that use the 1.50 kernel.

1. Copy the 150k_installer folder from the 150k_addon390v2.rar to X:\PSP\GAME
2. Copy the 150.PBP from the PSP folder in this archive to X:\
3. Run the application from the XMB, follow the on-screen instructions and you're done.

Enjoy your PSP.