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.