How to Downgrade from Dell system bios version A08 to A05 for Dell dimension 4550?
You do not, the version above A05 is written by Dell, not
Phoenix. Once installed, the Dell version rewrites the BIOS with a
different format and from that point on you are stuck with Dell's
version of the BIOS. Unless you buy another motherboard with an
older version of the Phoenix BIOS, you have to live with what you
have. Unless you can remove the BIOS Chip, you currently have and
solder an old Phoenix BIOS chip in its place.