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As Mike mentioned, if it is only the bottom flap gasket it runs in a channel along the outside bottom of the frame. A little Vaseline will help guide it all the way. Let the whole thing sit in the warm sun for a bit to soften the rubber and aid with installation. Pull tug push and pull and it will go. Have it extend beyond the edges of the frame for a better looking fit once remounted between the stanchions.
For the interior gasket surrounding the glass---- there are a couple of screws on each side to remove and the top frame section can be pulled out then you can remove the glass and fit the new rubber gasket. Best to check your glass at this point too for any imperfections and make certain it is SAFETY GLASS. It takes lots patience but you will fit everything back together. Good luck!!!