Using the Windows application to update your BIOS is not recommended at all as there's more of a chance that the process could go wrong. Always do it via DOS or the motherboard's built-in BIOS application - these are independent of OS type.
The option to update your board's BIOS will be in the BIOS > Tools Menu > Asus EZ Flash 2 option. If you need to know the full BIOS-update procedure, just ask.
But before doing it can I ask if there's a specific reason for wanting to update the BIOS in the first place? G