Open the door, and unscrew and remove the edge plate screws. Unscrew and remove all trim screws from the lock trim on both sides of the door. Turn the thumb turn counterclockwise and pull it from the lock trim.
Pull or pry the lock trim from the door with a chisel or pliers. Remove the mounting screws from the lock interior, and pull or pry the edge plate and bolt from the door edge. Spray a silicone-based lubricant into the keyhole, lock interior or edge plate if the parts do not loosen easily.
Brush debris from the lock assembly with an old toothbrush. Spray the interior of the assembly, keyhole and bolt with silicone-based lubricant and replace the lock and trim.
Measure or examine the gap between the top of the door and the trim. If the gap is not uniform or the door does not close easily, examine the hinges and replace loose screws with longer screws of the same or slightly larger diameter. Tighten all hinge screws.
Re-examine the gap between the door and the trim and between the bolt and strike plate alignment. If the bolt is too high for the strike, loosen the top or tighten hinge screws two revolutions with a screwdriver. If the bolt is too low for the strike, unscrew the bottom or tighten the top hinge screws two revolutions. Re-examine and adjust until the bolt hits the center of the strike.