How to replace a character model in LIS?

So I seen people do this and I want to try it. I’m about to have a new computer soon so I thouht while I still have LIS on the old one maybe I could try model replacement. I want to replace Cassidy’s model with another model but, how do people even do that? I seen somebody do it with Max’s model to this LIS. They never told how they did it. Does anybody know how you do that? When I look at the folder, I see there in paks files, so it makes me confused how to import aka replace a model with another.