Open the door and locate the screw on the edge of the door. It will be level with the base part of the cylinder ( do not unscrew any other screws along the edge of the door).

Undo this using a Phillips screwdriver  If the screw is stubborn and will not turn, apply oil and leave for a little while before making a fresh attempt.

Insert the key from the inside of the door. Turn the key slightly and slowly try to pull the key towards you. This should slide the barrel out. If you encounter resistance, the lock may be slightly misaligned, so turn the key slightly back and forth while pulling the key until you feel the cylinder start to move.

Slide the barrel out of the door.

Fitting the new or replacement barrel requires a simple reversal of the above process