how to change my main category names
You may find it useful when learning WordPress to review the official docs (codex) here:
https://codex.wordpress.org/Posts_Categories_SubPanel
Each post in WordPress is filed under one or more Categories. This aids in navigation and allows posts to be grouped with others of similar content.
Each Category may be assigned to a Category Parent, allowing you to set up a hierarchy within the Category structure. In creating Categories, recognize that each Category Name must be unique. Thus, even if two Categories have two different Parents, they must still have different names.
Links to your Categories are, by default, shown in two different places on your blog’s home page if you are using the WordPress Twenty Seventeen theme. First, WordPress lists these Category as links in your sidebar. Second, WordPress shows all the Categories to which a given post belongs under that post. When someone viewing your blog clicks on one of these Category links, an archive page with all the posts belonging to that Category will be displayed.
