Remove any food from the doors on the side of the freezer and from the front part of the freezer compartment as well. You want to give yourself a clear area to work with.
Grasp the old gasket by its interior flange and pull it back until you can see the gasket retainer. Use a hex head screwdriver to loosen the retaining screws running along the perimeter of the freezer door. Do not remove the screws, just loosen them. Once the screws are loosened, you can simply pull the old gasket out and discard it.
Work the lip of a new gasket behind the retainer, starting at one of the corners. Work the new gasket into the retainer one section at a time until the entire gasket is firmly in place then tighten the retaining screws. Lubricate the side of the gasket closest to the door hinge by applying a thin coating of petroleum jelly.