you need FPT Z170 Tool for DOS (with bootable USB)....
https://www.mediafire.com/?p66jhts7cfi26y8
download this first.
then make bootable usb stick with , in my example "rufus".
copy those files from link above to usb stick + bios file you want to downgrade.
i have used backup file which is already named "bios.bin", otherwise rename original downloaded file to bios.bin.
boot from usb and mine downgrade works just fine.
in my case , i was just downgrade from F20 to F20 beta which was working normal. I didn't try earlier versions.