Took ages to find these particular versions of "AFUDOS" DOS BIOS flasher for AMI BIOS, esp Asus mobos.
These versions allow BIOS downgrading...
You will need to put this on a bootable CD or USB drive along with your BIOS file, as the Bios file is over 2MB and will not fit on a floppy.
Now boot into Dos and type the following,
(xxxxx = your bios file name)
AFUDOS /ixxxxx.ROM /pbnc /n
AFUDOS /iP5KE602.ROM /pbnc /n
Make FreeDOS flashdrive for this purpose (tested, not just hearsay):