How do I change the default file manager?

[Tyrannocaster]

I know it's possible, because I did it a few years ago when I started using LL. I can't remember how to do it and searching the form has not given me an answer, perhaps because I didn't use the right terms, but I've tried several. I wanted to try Nemo, and it works but I have to launch it separately each time. I don't even know if I would want to keep it as the default because it's so clumsy to do this way.