How do I reset the Master Key on Mac Mini.?

I recently purchased a used Mac Mini from Gary Roberts and cannot find him, now. I want to upgrade my software, but to do so I need to know the Master Key password. Does anyone know how the Master Key password is reset.? I also need to change the hard drive ID from gary roberts to nancy brewer, my wife. She will be the user and main file folder USER. Any help would be appreciated. Don Brewer, 1PEdb.