If it is like my Asus M2N-SLI Deluxe, there are a 3 ways to do it. Also refer to the manual.
Bios-->Tools-->EZ Flash 2-->Proceede
Boot with 3.5" Floppy or USB Drive (bootable) with the EZ Flash sw on it and the new bios. Proceede from there.
Update within windows (I dont like this one).
Good Luck