Start by finding out what size lock you need. Measure the ‘backset’ of your door and the size of the hole in your door – backset is the distance from the edge of the door to the center of the door lock. It is generally 60mm but some doors have a 75mm backset. Some locks sold nowadays have an adjustable backset so can be either 60 or 75mm.
Remove the screws from your existing lock and pull the lock out. If you can’t see any screws holding the lock together, they will be under a cover plate which you can flick off using a flat screwdriver.
Take the screws out of the lock faceplate in the edge of the door and slide the old lock cylinder out.
Insert the new cylinder and screw the faceplate into place. If the holes are too big for the screws and they don’t grip in place put a matchstick in the hole alongside the thread of the screw, as this will increase the grip and help it stick.
Slot in the lock through the cylinder and screw into place.
Check the lock is working properly and the job’s done.