Helping people with computers... one answer at a time.
Well, unfortunately, the question itself shows a slight misunderstanding.
Believe it or not, the BIOS is unrelated to Windows.
The BIOS, or Basic Input Output System, is a part of your computer, not a part of Windows. It's software that's stored within flash memory chip on your computer's motherboard. (Flash memory is normally read-only, but can be reprogrammed with appropriate tools.) Windows, Linux ... or no operating system at all, your BIOS is there as soon as you turn on the machine.
Unfortunately exactly how you update the BIOS varies from one type of motherboard to another, so I can't give you specific instructions. Typically it involves downloading the new BIOS image and an installation program, often in the same file, copying them to a bootable floppy disk, booting from that floppy, and running the installation program.
But it might be different for your computer.
So the best advice is simply to check with the manufacturer of your computer or motherboard. Most likely their support web site will include both the updated BIOS and the instructions you need on how to install it.