- Drill out the old chamber. This will be much easier if you use a drill press.
- Open the lock. Sometimes this is really easy, other times you have to whack the lock against something hard so the pins will fall out. Try using a flathead to turn the chamber.
- Locate the screw hidden in the shackle hole.
- Unscrew it.
- Remove the retainer plate.
- Make sure you have all the pieces: the retainer plate, the screw, the nut, and the old chamber. You can discard the old chamber.
- Insert a new chamber, and reassemble. Make sure the new chamber is oriented as shown.
Well, it depends on your scenario. If, for example you have a key and lock, but your lock is otherwise damaged (e.g. shackle has been cut), you can transfer your chamber to another lock for which you don't have the key. Chambers can also be bought in bulk from the manufacturer.
Thanks to John at 123 Tompkins!