I have an '81 300TD-T and I just swapped out my driver side for a passenger side. I assume that your 240D is the same...if anyone disagrees please tell me so.
The difficulty lies in if you want to swap out the entire seat or just the cushion.
If you have another seat, the entire frame is only held to the car with 4 bolts. 2 in front and 2 in back. I had to push my seat all the way to the front, and then adjust it all the way up to get to the rear 2 bolts. Mine had a seatbelt buzzer wire attached so be careful there. Then, the seatbelt clicker (that is the technical term for it
) is attached to a long bar that is attached in the back seat floor area. Just slide it off and pull it out.
If you want to get just the cushion off the frame so that the seat adjusters, etc. all stay with teh car, let me know 'cause that is what I had to do and it is more in-depth than what I can tell you here...
Hope this helps!