There is a nut on that stud that you need to remove. Not the nut you see, but one on the inside of that box that's welded to the inside of the door frame. If you look thru the hole in the door frame (right behind the where the interior door panel covers up) you will see a nut on that stud. The stud has a slot in the end of it. Remove the nut, sometimes when the threads get dirty or boogered up the stud will turn, but try to get the nut off without doing that because it will change your window alignment. It may need to be changed of course, but it's always nice to start where it was, so try not to let the stud turn.
Anyway, you remove that nut that is thru the hole in the door frame and you tilt the wing window in at the top letting that stud come out of the hole in the door. You then pull the wing window up out of the door. You will need to twist it of course when that stud on the bottom of the channel reaches the top of the door.