To add or remove categories available to a team simply click on your Organization name in the top left corner, navigate to Team Management and press ⋯ to Edit of the specific team. You can then edit the categories linked to this team.
When adding categories you can either choose one of the default categories, or add your own. Simply type the name and press Enter.
When removing a category, it will not be available anymore to be assigned to a request, however, the removed category will not be removed from existing requests.
Don't forget to press Save changes once you're done with the editing.