PSP Firmware update

Recently my colleague request me to update his PSP which was at that time having custom firmware version 3.4 M33. I have done this before for my PSP, but that from 3.7 M33 not older. So I browse internet then I found out that you need at least version 3.52 and above. I went to dark alex website http://dark-alex.org/ download the 3.52 version and also download the eboot (original PSP firmware) and rename it to 350.pbp
Steps:

1. Extract file downloaded from Dark Alex to your computer
2. Connect PSP to Computer via USB. Go to USB Setting (in PSP)
   
3. copy UPDATE folder (in computer) into /PSP/Game (in PSP)
4. copy 350.pbp (in computer) into /PSP/Game/UPDATE (in PSP)
5. In PSP, go to game and run the updater. Then Reboot.
6. Then I can upate to newer version repeating steps 1-4. but using its corresponding files. i.e. version 3.9 M33 using 390.pbp and so on
   

Latest version: 4.01 M33-2
http://dark-alex.org/