You can reboot the FortiGate unit from the backup firmware, which then becomes the default firmware.
Starting from the main BIOS menu
[B]: Boot with backup firmware and set as default.
If the boot device contains backup firmware, the FortiGate unit reboots. Otherwise the unit responds:
Failed to mount filesystem. . .
Mount back up partition failed.
Back up image open failed.
Press ‘Y’ or ‘y’ to boot default image.