You'll need to have the case open to do this. If you've lost the key, your options are picking the lock or drilling it out. Givi recommended a 1/4" drill bit, but I opted to not drill the lock out. A locksmith should be able to open the case.
Step 1: What you need
You'll need a Phillips screwdriver. A small flat screwdriver or something of that shape will help with the circlip, but you may be able to do without it.
If you have the tail light kit installed, you'll need a 2.5mm hex wrench.
Step 2: Remove the tail light plastic insert
Step 3: Unscrew the back of the locking assembly
Step 4: Remove the locking assembly
Step 5: Remove the circlip
Step 6: Replace the surround and lock cylinder
Step 7: Replace the latch and circlip
Make sure the lock is in the open position and replace the latch in the open position. Or make sure the lock is in the closed position and the latch is in the closed position. Your choice. Press the latch firmly into place, so that you can see the groove in the end of the lock cylinder. Slide the circlip into the groove.