Motherboards come with a BIOS version already stored in its flash ROM.

You usually don't necessarily have to update the BIOS on a new motherboard. Sometimes you do, but that is only when the motherboard BIOS is older than some new version of a component, such as to accommodate a new CPU family.

You can check the motherboard mfg. site for new BIOS versions for the motherboard and what the update does. Download the latest version from a working computer and put it on a floppy.

The procedure for installing the new BIOS version is in your motherboard manual and usually described on the website from which you downloaded the newer BIOS version. In general, you have to start your computer and enter the BIOS settings menu to update the BIOS. Alternatively, some BIOS'es are updated by booting to a floppy with a program that updates the BIOS. Again, check the documentation that came with the motherboard and the BIOS update.

After you have updated the motherboard BIOS and safely exited the BIOS settings (or the BIOS installation program), turn off your computer. Turn on your computer, quickly place the OS installation CD into the CD drive, and follow the procedure in the OS installation program that automatically runs.

What OS?
What motherboard model?