The ONLY way to downgrade a 3.90 firmware PSP is to use some version of Pandora's battery. From here there are two options:
Softmod: THIS REQUIRES A CUSTOM FIRMWARE PSP! You need a friend who has a custom firmware PSP or you can purchase one online. If you have a buddy, creating the actual battery is quite easy. You just download some programs and files and viola.
Hardmod: THIS REQUIRES BALLS AND SKILLS! This requires opening up your battery and I probably wouldn't recommend this for you.
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