If you see a plastic membrane, that's stuck to the door frame with glue. You have to carefully unpick it. It stays sticky, so you CAN replace it afterwards. It's required, to stop any water that runs down the window from soaking the door cards, so don't not refit it.
The lock cylinder is held in with a U clip. You just pull it out and that makes the lock cylinder loose in the hole (the U clips fits into a groove in the lock cylinder body).
The lock cyclinder comes out through the outside of the door (though you have to disconnect it from the arm that connects it to the lock mechanism first. That is just a ball joint but you will not believe how tight it is.
The cylinder is held into the lock barrel by a circlip at the back. be careful not no break the plastic outer sheath when you dismantle it.. and don't lose the little circlip that locks the cylinder into the barrel.
The new cylinder just snaps in to the lock barrel (you push it in until it clicks into place).
Ralf S.