There are some clips that need to be pulled from behind the panel. I think there are two on each end if I remember. You can use a special door panel tool or build one by taking a stiff 1" putty knife and grinding a slot down the middle to make a fork. Do NOT use a screwdriver unless you plan to buy new panels. Pry the panel forward with the tool and bow the panel forward enough to clear the window crank stud and panel clips and slide the panel to the rear to remove. Be very careful not to brake them or tear the vinyl. You can slide a large wood dowel (rub one from a closet or use a jack handle) horizontally behind the panel to help maintain the bow while you slide it off.

You might need to replace the vapor barier while you're at it. Go to Lowes and get some clear plastic sheet (the stuff you use in crawl spaces). Cut to fit and attach with some 3M Strip Caulk.