I'm going from memory here, so I might be a little off. I think you turn the key part way, and insert a paper clip (or similar stiff wire) into the hole on the lock cylinder. This holds down a spring-loaded pin. Then turn the black collar that surrounds the silver lock cylinder face CCW, it just unscrews. You may need to 'start' it with some pliers if it's stuck, then it should just spin off, and the cylinder comes out when the collar is off. It makes more sense once you see the innards.

Installation is the reverse, but when the collar is tight, you may need to loosen it a partial turn to remove the key (to let the key return to the 'off' position.)<br>
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Have fun!<br>
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-DM