Blind flash it if you don't have a spare pci video card. I don't remember the exact steps. You'll need to google it.
But you boot from a usb stick with the bios on there and then wait a moment to make sure the computer booted up in dos and then type out the commands that would've flashed the card and wait 5 minutes and then reboot.
Does anyone know why the cmd method doesn't work?
It worked for 6950 -> 6970 but not in reverse?