It's not that simple as a downgrade. you need to downgrade to an actual Custom Firmware (CFW). How you go about it depends on your model. Unless you have an orignal model you'll need to find someone who can make you a Pandora battery, most people with a hacked PSP can make these as it's a standard feature on most of the current CFW. Then you'll need to set up a memory card to flash your firmware back to custom firmware, and from there you can update to a newer CUSTOM Firmware. The only CFW tat can play current PSP releases is 5.50Gen-D with uses a bypass which prevents the games from seeing that you have an older firmware version. Other versions can play the games that it requires hacking the UMD isos with a modified EBOOT and LBA, which sounds a little beyond your level given that your PSP was 'somehow modded' I'm guessing that you're not familiar with the process if modifying UMD Isos.
If you need specific information, or to find someone local to you that can help you with the Pandora Battery and downgrade software, you can check most PSP hacking forms, PSPmod has a decent enough tutorial on how to do it yourself, and even some automated programs to do it for you once you have the Battery. Keep in mind that with any firmware modifications, it's always possible to brick your system, so if the person who flashed your PSP in the first place can help you, all the better.