The only dumb question is the one not asked.
There is a plastic strip stitched into the bottom part of the door upper. It is on the outer part at the very bottom just inside the fabric. It hooks under a metal lip fastened to the lower part of the door. You have to sort of pull it down and out or peel it out. It isn't easy. I like to start at one end and work my way down. It is easier than starting in the middle, I think.
Let us know if you need any other help.