It's quite simple.
If you know which bios version you were originally having on your motherboard, you can simply go to the download page for this board and search for all available downloads for this particulare board. By this way you should be able to see the latest bios (the one that you have) as well as previous revisions of bios. Simply download the version you want and perform the downgrade the same way you performed the bios update. It's simple as that.