Yes, you can definitely downgrade.
Please note you can not downgrade from the WebUI.
You need to have TFTP server reachable from the firewall (preferably local to firewall). Load the ScreenOS (make sure to unzip the file) to the root folder of the tftp server
On the firewall, via CLI run the following:
save soft from tftp <tftp IP> <filename> to flash
Once the entire software has completed, reset the device:
reset
If you have console access, that would be best as you can monitor bootup process from there.