Pull the window out. Scrape the old window seal off. Be sure to get all the old gasket off and get all the old sticky off. Use rubbing Alcohol to clean the surface and follow with a dry whipping cloth to get the residue off. Go to a place that sells canopies/shells and buy enough to reseal the window. Start by sticking the new gasket on the bottom of the window frame, attach all the way around the window overlapping an 1/8 of an inch from the starting point.
With the new window seal stuck in place run a bead of silicone all around the inside edge of the new seal. Be sure when you run the bead that all parts of the new seal and the inside edge of the window frame are making contact with the silicone.
Be sure to use 100% silicone that latex crap is garbage. After you run the bead of silicone set the window in the hole of the shell and have someone hold in place till you get the screws and clamp ring back on. If you do it this way you won't have a $hithouse mess on the out side of your shell. It will never leak between the gasket and the shell. Good Luck