It is nice that you are understanding the WordPress day by day.
You can change the category name via `Posts > Categories`.
Category name will display on the theme archive pages. Archive pages show the posts that are tagged or categorized in them.
Their headline will show like,
Category: Category Name
Tag: Tag Name
Similarly, you can edit or create new posts via `Posts`
And pages via `Pages`
Following article will also be helpful to you.