With the door open look for a nylon "keeper" ring in a groove in the middle of the seal face. (It will be slightly below the level of the rubber seal itself) The keeper ring has a small gap, usually at the top. Insert a small screwdriver or knife blade into the gap and pry out the keeper. Then pull the door gasket out starting at the top. Reverse the procedure to install the new gasket starting at the bottom and working up both sides to the top. Insert the keeper into its groove starting at the bottom and working up both sides. Make sure that the gasket is fully seated before trying to install the keeper. It will take some force to put the keeper in place; liquid dish soap will help.
Richard Seymour, April 2008