1) Click 'Manage Categories' under the 'Lists' heading of the 'Control Panel'.
2) Under 'Existing Categories:' select one of the categories you'd like to combine from the dropdown menu.
3) Click the 'Edit' button
4) Click the 'Advanced' tab.
5) Next to 'Combine Category w/' select the category you would like to combine with your original selection from the drop-down menu.
6) When you are finished, click the 'Save' button.