On my '84 Westy I removed the seat back and the rails, then fliped the
seat bottom upside down and pushed on the seat to compress the foam.  The
upholstery is held in place by some thin cardboard strips that are stiched
to the upholstery and fit into a grove on the underside of the seat.  So
just compress the foam and pull on the upholstery towards the bottom of
the seat as if you were trying to actually streach the upholstery over the
seat more than what it is.  The cardboard strip should come out of the
grove and the upholstery can be teased off gently.

Pascal
'84 Westy