I'm afraid a bit more info is needed. What brand PC is it? Or if a clone PC, what mainboard?
If you can find the make and model of board (including the revision number if at all possible) you can go to the manufacturer's website and download an updated BIOS. These usually just install from a DOS prompt.
But also, can I ask why you want to flash the BIOS. BIOS upgrades often simply fix small bugs or add minor extra features. Is there some specific problem you have with the machine? If not, it's proabably best left alone.
Jim