The following example was done on a FortiOs 6.2.2 VM. It is expected that a configuration backup file is available.
Log in to the web interface of the firewall:
On the top right corner, go to admin > Configuration > Restore:
Click “Upload”, browse and select a configuration backup file, then click “OK”:
Click “OK” to confirm reboot:
Once the reboot is complete, login and check if service is restored.