You can move an existing category. E.g.: If you want a subcategory to be a main category.  Or you move a category to another category. 

Therefor please proceed as follows: 

iOS , Android and Windows 11 / Amazon FireOS Version:

• 'Settings' > 'Categories & Rules'
  Tab on the main- or subcategory > Edit > scroll down >'Move category'.
  Then select the main- or subcategory you want to move the selected category in.

macOS-Version: 

• 'Options' or 'Settings' > 'Categories & Rules'
  You can pull the main- or subcategory via Drag 'n Drop to the new place