You can only use web version to customize your categories. On the app, you can only view existing categories.
Please follow the below screen instruction to add or edit a category and maintain its sub categories.
Please note that if you have selected USA or Canada as your country during the on-boarding process, you will see the approved category list for small business by IRS or CRA. You cannot change such categories, but you can add your own categories or modify categories you have added.
The category edit screen looks like the following: