Mortise lock is the name given to the large, rectangular locks that fit into a slot, or "mortise" in the edge of the door. These locks were common at the turn of the twentieth century and are still in use today. The cylinder lock is the modern standard. The best example of a cylinder lock is the deadbolt on your front door. You will need to fill the void left by the mortise lock before proceeding to install the cylinder lock.
Things You'll Need
Removing the Old Lock
Remove the screws attaching the inside face plate to the edge of the door as well as the plates on the door faces. Use a screwdriver to remove all screws rather than a drill. Remove the face plates, prying them up with the screwdriver if needed. Pull the lock out of the edge of the door.
Measure the depth, width and height of the lock mortise. Use a table saw and miter to cut a block of wood to fill it. Coat the block with glue on all faces that will be inserted inside the door and slide it in. Nail through the door with a pin nail gun and 1-inch nails, one nail at the top and one at the bottom of the block to hold it until the glue hardens.
Fold the paper template that came with your lock over the edge of the door at the appropriate height. Typically deadbolts are installed at 42 inches on center, but align it with the lock bolt hole in the door jamb for ease of installation.
Installing the New Lock
Mark the center of the holes for the face and edge of the door, poking a pencil through the paper template to create the mark. Bore through the face of the door, centered on the mark, with a 2-1/8-inch hole saw. Apply steady pressure, but do not force the saw, allow it to cut through the other side to minimize splintering.
Bore the lock bolt hole aligned with the center mark you made on the edge of the door with a 1-inch paddle bit. Drill through until you reach the round mortise hole you made in the door face.
Center the bolt collar plate around this hole and trace its outline. Cut the material from the outline with a sharp wood chisel to a depth of 1/8-inch. Slide the deadbolt lock mechanism into the edge of the door and install the two mounting screws. Fit the halves of the lock barrel through the holes in the mechanism and install the two mounting screws inside the door.
Remove the striker plate from the door jamb. Fit the new deadbolt striker plate in place and install the two mounting screws. Close the door and lock it to test. Adjust the striker plate as needed for a good fit.