I own a PSP 2003 model & it's currently on 5.50 GEN-D3 Prometheus-4

It's been quite a long time since I've messed 'round with PSP custom firmwares, psp hacking & whatnot.. And, I've noticed new custom firmwares being released like 6.20 & 6.35 so, I just need to know if I'm able to update to a newer custom firmware at all..? And if I can update to a newer one, Can you give me instructions on how to do so.. 'cause last time I tried to update a newer firmware.. it kept telling me that the firmware version is v9.99 & I couldn't update..

So, Please help if you can

Thanks