If you're honestly desperate to try the game in such a smoldering state, you'll need to know the PSP you have, and the OFW that you have installed. I have a PSP-2001 with 6.20 installed. I simply searched a how-to and got a guide that shows me how to install and run the CFW software from a Memory Stick PRO Duo (or a variant that fits the port) and my PSP now runs 6.20 PRO-C2.
After that, find the PSP port of SRB2, dump it on the Memory Stick in X:\PSP\Games (where X is a drive letter) and make sure you've got the SRB2 data files copied over.