Replacing the weatherstripping at the top edge of the door is not difficult, however there are some tricks. You will need an electric drill, a grinding tool like a dremel tool (with a flexible shaft, if possible), some adhesive remover, some trim cement (I prefer 3M products here), Some WD-40 or equivalent lubricant, stainless steel pop rivets and a pop rivet tool.
The first thing you have to do is remove the old weatherstrip. To do this, you will have to drill out the rivets holding the strip to the top of the door. Before you do this you MUST protect the torsion springs. Cover them with several layers of heavy duct tape. The spring could shatter if you score it with the drill bit. Take care to only remove enough of the rivet to pop off the head. You don't want to drill down into the stainless backing as you will reuse it. Once all of the rivets have been drilled out, carefully remove the old weatherstrip by sliding it toward the back of the car and then pulling the front of the weatherstrip from the door. Be careful not to bend the stainless backing. If you have some trouble, spray some WD-40 between the door and the rubber. This will allow the strip to slide easier.
Once the old weatherstrip is out, carefully examine the old rubber to determine how the it is attached to the stainless. Then select the new replacement rubber. The curved part of the rubber should face up. Remove the old rubber and clean off ALL of the old adhesive. The more you remove, the better the new adhesive will work. Next, glue the new rubber to the stainless backing, following the directions on the adhesive you choose. When the adhesive has cured, punch holes in the new rubber at the holes in the stainless backing.
While the glue is drying, use the grinder to grind off as much of the old rivets as you can. It is important to remove as much of the old rivet as possible, as you are going to push the remainder back into the door. If you leave too much, you can leave a little dimple in the outer skin of the door.
Now that you have the new rubber bonded to the backing and the old rivets are now rolling around in the door, you are now ready to reinstall the weatherstrip. Insert the new assembly the same way you removed the old one - push it to the rear first, then back to the front. Remember that you can use WD-40 to make things move a little easier. This part can be difficult, so take your time and walk away if you find yourself getting frustrated. Line the holes you punched in the rubber with the rivet holes in the top of the door. I like to start in the center of the weatherstrip and work out to both ends, alternating from front to back. This keeps the weatherstrip snug to the door without risking kinks. You need to be careful with the pop rivet tool to make sure that the barrel of the rivet catches the hole in the top of the door. This means that you really have to push on the rivet while you are working the tool. You should see the rubber deform a little as the rivet grabs the door. If it doesn't, you will have to drill out the rivet again and use another one.
I don't recall the precise size of the rivets, but they are a common size and can be had from DeLorean One (and I'm sure the other parts suppliers), or you can get them from a fastener company like Fastenol. I did not have much luck getting stainless steel rivets from an auto supplier. They typically carry aluminum rivets, which will discolor over time.