I tried this once (on a car in a scrap yard, I might add - not on my own car!) And its a massive headache!
You CAN get the thing off, its a matter of plastic edging clips, as i recall, and careful / pragmatic prying, then you'll note that there are hooks integral to the seat cover, further inside, which attach to a wire frame, part of the main body of the seat. IF you can get these off, some special tool like an adapted flathead driver, say, may work. Then you can get to the heated pad. This is attached to the seat, too. I forget exactly how, hooks or clips.
But as for getting the seat cover back on, I think an unholsterers is your best bet. I'm not trying to put anyone off doing this, I just wouldnt recommend attacking your own seat. HOWEVER, you can always get a seat from a scrap yard (I bought a leather Saab seat once for a tenner!) and play merrily to your heart's content, and learn by your mistakes. I'd love for someone to do it, frankly, as its so-far flummoxed me!