The headrest must be removed first, to allow proper access for upholstery removal. It simply pulls (with some difficulty) straight up, and out of the seat back. Don’t be scared to apply a little muscle.
(NOTE: If you’re replacing the upholstery as well as the foams, cut out the rear portion of the vinyl seatback, remove the back-board and push in the metal tab on the headrest shaft to remove it easily.)
While you’re at it, remove the Phillips head screw and slide off the chrome seat adjuster handle. It too needs to be removed, before the upholstery can be slid off.
You must remove the Philips headed screw on each lower “flap” on the seat sides.
These screws retain the lower portion of the upholstery onto the seat back frame.
This photo (taken from the rear) shows the spring steel clips that must be pried off with a screwdriver.
This will allow the seat back upholstery to be pulled up, off the seat.
After removing the upholstery clips, open the seat up fully (you will need to temporarily reinstall the chrome seat handle for this).
With the seat fully open, grasp the flap of upholstery material and feed it up and out of the “crack” in the seat. The seat back upholstery can then be slid off. It will need to be fed over the seat adjuster shaft though.
Similarly to the seat back, the seat bottom upholstery is secured on all four sides by round, spring steel clips. Care should be used when prying off the clips.
The ends of each clip are “barbed” to hold them in place. Carelessly prying them off is sure to damage the vinyl upholstery material. (Probably this does not matter anyways since you are likely replacing it with new).