The Best Way to Downgrade any PSP
Dangers of downgrading
Know that downgrading your PSP voids the warranty and can be quite dangerous to your hardware.
Downgrading often requires upgrading: You can only use a downgrader that goes from one firmware version to another, so most users will need to update their PSP to at least version 2.0 (USA) in order to then downgrade it to 1.5 or 1.0.
Find your firmware
Determine which version of the PSP operating system you are currently running:
Navigate to the main menu on your PSP.
Select Settings -> System Settings -> System Information.
Look for System Software Version X.XX. Where X.XX is the version number of your PSP firmware.
Now you will know which downloader and/or updater you will need.
Note that any version greater than 1.51 will prevent you from running Homebrew and other independent applications.
Use the tools
Be aware that your firmware updater and associated tools are located on the Memory card.
Know that there are many tools to downgrade your PSP, depending on the version number of your firmware:
Version 2.0 and 2.01 can be downgraded with Easy Downgrader.
Versions 3.0 to 3.03 can be downgraded
Search for other downgraders - HERE.
Get the remaining Information About How to Downgrade a PSP - HERE