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.