You can downgrade it with signed firmware but it looks like Bitmain always remove older version from their download page.
Maybe some user here who use the same miner has a backup or older firmware version but I don't recommend to download them from unknown source unless if they have a bitmain direct link of older version.
if you mean using the upgrade process and choose the old firmware i already tried that and got this error (RUNME:cannot downgrade!!!) system upgrade failed , is there a way to bypass that