In the data entry window, simply right-click the Category field and choose Edit Table from the popup menu:

(To simply add a new category you can also choose Add, or to edit the selected category choose Edit.)

You see the 'Edit Lookup Table (Category)' window. From this window you can add, edit and delete categories:

Another option is to use the Lookup View: Select Category from the View button to open the Category table in the Lookup View.