This explains how to replace the lock on a Givi hard motorcycle top/side case. The specific model shown is an E55 Maxia 3 with the tail light kit installed. You might want to do this because you bought your top case and side case separately and would like them to use the same key, or because you've lost the key to your case. Givi doesn't seem to sell replacement keys, but they do sell replacement locks at a reasonable price ($10 each or $30 for a matched set of 3).
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 new lock. The kit comes with a pair of keys, a new lock cylinder, a surround (housing for the lock) and a circlip.
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
If you have the tail light kit installed, you'll need to remove the plastic insert that holds the contacts on. You'll need to remove two screws with a 2.5mm hex wrench.
Step 3: Unscrew the back of the locking assembly
There are four Phillips screws holding the locking assembly on. Unscrew them.
Step 4: Remove the locking assembly
It should just pop out of the case, now that you've removed the screws.
Step 5: Remove the circlip
Remove the circlip from the back of the old lock cylinder. The latch should now be removable. There's a ball bearing behind the latch. It'll probably fall out at this point. Try not to lose it. The old lock cylinder should now slip out of the front of the surround, and the surround should slide out as well.
Step 6: Replace the surround and lock cylinder
Slide the new surround and lock cylinder into place from the front of the lock.