Parent and Child Theme

Hey guys,

I'm a little confused. I modified the following files:

edu-clean/_inc/css/base.css and edu-clean/header.php

I know they'll be overwritten if there's an update. I'm not sure how to use the child theme edu-clean-child to avoid this if at all possible. Is it for individual div classes that I modify to be put inside edu-clean-child/_inc/css/child-style.css file ?