If you’re on 5.00 M33:
* Turn off your PSP and holding the R trigger when turning it on.
* Go to Configuration > XMB USB Device and change it from Memory Stick to flash0
* Go to Use version.txt and select the option to enable it.
* Go to Back > Exit to restart your PSP
# Connect your PSP to your PC

Important Note: Be careful with this step. You are editing flash0, it’s where the firmware of your PSP resides. If you delete or modify anything aside from version.txt you will BRICK YOUR PSP. Be very careful.
# Browse your PSP and go to folders VSH > etc. Inside you should see a version.txt file.
# Right-click the version.txt file and uncheck the Read-Only property.
# Open the version.txt file. You will see:
view sourceprint?
1.release:5.00:
2.build:2740,0,3,1,0:builder@vsh-build6
3.system:46876@release_500,005000010:
4.vsh:p6264@release_500,v46652@release_5
…
5.target:1:WorldWide
# Replace it with
view sourceprint?
1.release:5.50:
2.build:3062,0,3,1,0:builder@vsh-build6
3.system:50781@release_550,0x05050010:
4.vsh:p6399@release_550,v50953@release_5
50,20090415:
5.target:1:WorldWide

and save.
# Skip this step if you’re on 5.03 GEN-A or Custom Firmware Enabler: restart your PSP.
# Press and hold select to access the VSH menu. Change back the XMB USB Device
# And you’re done