Atrium locks are mortise-style locks, which fit into the edge of a door. The lock trim is usually large and decorative, with a thumb-turn lock located below the levered handles. Only a few holes must be drilled on the front and back panels of the door, so the locks can be replaced without having to patch large holes. You can also replace the lock cylinder and keep the old trim if you need to rekey the lock.
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Remove the strike-plate screws and strike box from the door frame using the Phillips screwdriver.
Press the small release button on the door handle post with the tip of the screwdriver and remove the handles.
Remove the screws from the lock case on the door edge and the screws from the trim around the handles and lock using the screwdriver.
Remove the thumb turn and lock trim.
Pull the lock case from the edge of the door.