Tesla mobile service came out last week and replaced my rear seat bottom cushion due to some odd wrinkles at the leading edge of the seat behind the driver's seat. I actually helped the technician do this. The seat pops out very easily via two small manual switches/levers at the front edge (bottom) of the seat cushion. Getting the new seat in is a bit of a pain, but it went in just fine with some grunting and pushing.
I was surprised by how lightweight the seat bottom was when we had it out of the car. Like maybe 10-15 lbs - easy for a single person to carry around with no problem. Certainly way lighter than what I expected.
One funny thing I noticed when we had the seat out. There was a big dark footprint clearly visible right in the middle of the white insulation material under the rear seat. Someone from the factory had stepped there with dirty shoes at some point while the car was being assembled. The tech was sort of embarrassed by this, and offered to clean it off, but I said "no thanks." It's just classic Tesla manufacturing shoddy procedures, and I'll never see it again.