If the door is in the open position, on the edge of the door you'll see a screw head, roughly in line with the door lock barrel. Remove that retaining screw, the put your spare key in the lock and turn it slightly until you can slide it out. Then put the new barrel in the same way, with the key in the lock, turned slightly to allow barrel to slide into place, then line up the retaining screw and screw it back in.
Alan, April 2010