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.