There are a number of things you can do on Manage Categories:
1. Re-arrange your existing categories by dragging and dropping them in your desired position
2. Editing your current category by clicking on the drop down icon 'v' and clicking on "Edit"
3. Deleting categories that you no longer want to use by clicking on the two horizontal dots and choosing "delete"
To learn how to create a new category, read this article: Add a category
To learn how to create a sub-category, read this article: Add a sub-category