Just replaced the locks on an Atlantis 200 which should be the same.
Pop off the 3 rubber caps around the lock and drill out the rivets.
Inside the box pull the plastic unit away from you (bending the aluminium strip slightly but this is fine) gaining access to 2 screws. Unscrew these (I used a small ratchet with a pozi bit rather than a screwdriver) and remove the plastic unit.
You'll have to take out the lock cartridge next. Theres various wedges in the plastic which you should try to hold open then a flat screwdriver near the cog should lever the whole thing out.
To get the barrel lock out of it's cartridge you'll have to push the tumblers in from the back with a very small screwdriver and gradually work it out. (the one I did had a snapped key so I'm assuming this part is easier if you are just changing locks for keyed alike purposes)
Fitting the new lock in to it's cartridge should be easy enough. When you reinsert the cartridge in to the housing make sure that the sliding bolt (that your key turns) is fully in the open position, otherwse your key won't do a thing.
Screw the housing back on and re-rivet the points you drilled.

Dan (Rothwell Trailers)