I've rebuilt many Tesla seats but I haven't replaced the entire upper seat cover. I've replaced many lower or bottom cushion. I have removed the lumbar support and put in my own foam. I did that entirely from the back. I pulled the head rest and popped the black plastic in the back. I'd be more concerned about the air bag and safety. There's an airbag in the outside of the seat upper, facing the door. It's the side impact airbag. I would shut the car down entirely. I'd remove the four bolts to the floor to gain access underneath. I'd disconnect all the connectors to the seat upper. I'd lean the seat upper forwards.
In carefully watching the videos, I can see the upper is held on similarly to the lower.
There are “clips” that hold the faux leather covers to the metal frame. The seat upper faux leather doesn't encase the entire metal frame, instead just has a front and clips onto the metal.
I'd remove the black plastic, following the videos, then unclip the clips around the metal frame. There might be one on the bottom too.
If you bought an upper complete with foam, then remove the entire foam plus the faux leather cover from the frame. Then install the new. If you only bought the faux cover, you'd have to pull the faux cover off the foam. There are clips and Velcro involved.
Videos:
This guy shows how to remove the headrest and the back plastic.
“Tesla Model 3 removing front seat back on newer cars”
Also shows more details in removal of the seat back.
“Tesla Model Y/3 Carbon Fiber Seat Back Install”
