How can I fix my Admiral freezer door so it seals?

I have an Admiral refrigerator/freezer, model LTF2112AR. The freezer door sags and the magnet seal on the top left side and about half way across the top of the door does not come up to the metal cabinet and thus does not seal on the top left side and part way across the top. The door hinge is on the right of the refrigerator/freezer. Because it does not shut tightly, then the ice melts in the top of the freezer cabinet and drips on the bottom of freezer and builds up ice.