you want to do a firmware update on the bios to use a different processor ? i don't know about the motherboard but I'm going to read on it later. Usually if the board doesn't support it then it wont work, what kind of pentium do you have ? some of the sockets are different for Pentium and c2d's
You will have to go to the web site of the motherboard manufacturer and download a bios flash (usually on to a floppy). Make sure you understand and follow the directions, flashing the bios can be a tricky experience. Good luck with it.