Remove the door panel and remove the catch mechanism. As an extra, you'll have to remove the 3rd cable.
It's this cable that connects the latch mechanism to the key operated lock located just above the handle.



Drill out the rivet just below the lock, twist the lock body anticlockwise by
45 degrees and push it into the door. Reach underneath and pull the lock out.



The new lock will obviously need to be riveted back in place.