On the left side rear fender just behind the red reflector there is a key slot. That will release the seat. Sometimes the seat will have to be lifted and pryed while the key is turned.
Rich K ( Rune Ski )
the amount of lifting and prying depends upon how well the Corbin seat fits into the bail retainer locking gizmo. Some of us have had that bail (loop of metal) actually break away from the seat. I had to fiberglass mine back onto the seat.