Lift the top a little at a time and use a screwdriver inserted 90 degrees (pointed inside from the outside) and slide it along to seperate the gasket from the sides. same to seperate from the top. scrub it up with westleys bleche white and a stiff nylon brush, coat with decent rubber protectant. i didn't glue mine on, just started all the bolts loosely until i was sure it was set and watched it when i was tightening down. the gasket has little ribs in it to keep the moisture out, so it doesnt' need glue and i didn't want something that might suck the moisture from the rubber.
definitely scrub it up tho, it will make it nice and pliable. when they get stiff and crunchy, they will leak. had that issue in my 71, water was pouring down the corner windows. cleaned that gasket and reassembled and never had an issue again.
ige tlca #1431