In order to restore Syabas firmware, the ".dff"-file containing the image of Syabas firmare is needed. Such a file ("dune_firmware_syabas_backup.dff") is typically created when replacing Syabas firmware with RusHD firmware (see "Replacing Syabas firmware with RusHD firmware").
Use the file with the image of Syabas firmware ("dune_firmware_syabas_backup.dff") to update the firmware exactly as when updating to another version of RusHD firmware. See "Updating RusHD firmware".