Prop open the refrigerator door with something solid, such as a large can or heavy box.
Unscrew the retainer that holds the gasket in place using a Philips screwdriver. There will be a number of screws circling the outside of the retainer. Turn the screws counterclockwise just enough so the gasket becomes loose. Do not remove the screws, just loosen them.
Straighten out the gasket if it has turned or flipped over in some areas. Slide up any area that has moved down in the retainer leaving an open space and bulging gasket. If the seal is broken or cracking, remove it and insert a replacement gasket.
Tighten the screws in the retainer to secure the gasket. Turn the screwdriver clockwise until the screws are tight.