If it's HOBBY you just remove the # from the aircraft since YOU'RE the one registered not the aircraft.
If it's COMMERCIAL it depends on HOW you registered it. If you did the Paper Trail method you'll have to get the forms from the FSDO to cancel it (again in TRIPLICATE). If you did it via DroneZone you just go back to DroneZone and CANCEL the registration there. I've done it both ways.
Allen