Updating Bios Problem

I wanted to update the bios on my old motherboard "Asus P5B" https://www.asus.com/Motherboards/P5B/#support

but when i try to select an OS it only goes up to windows 7 64bit, I have win 8.1 and I can't select it. 

Will it hurt my motherboard if i try to install the win 7 64 bit version or will it brake it ? 



The OS doesn't affect the actual BIOS file. Just the utility within the OS to update the BIOS. I always recommend updating the BIOS through the BIOS first if your BIOS (and most do) allows it.