There are a number of various models in the P2B series from Asus.
Please refer to
this link for more info.
Basically you need to get the exact model from the motherboard, it's usually stamped in between a couple of the PCI slots.
Then refer to the link above and find the exact model and see if there is a firmware update. Download this along with the flasher utility and then format a blank floppy disk to copy the flasher and the updated BIOS image to.
Boot from a DOS bootable disk and there you go. Asus usually gives a very detailed step-by-step procedure on how to do this as well
Here's the direct FTP link. You can use a web browser or an FTP client to access this server.