I found the answer at

http://forums.oscommerce.com/ - Direct Link

To change the box heading of categories open your catalog/includes/languages/english.php file and look for the following piece of code

// categories box text in includes/boxes/categories.php
define('BOX_HEADING_CATEGORIES', 'Categories');

Change the Categories to what ever you want it to be so it would look like

// categories box text in includes/boxes/categories.php
define('BOX_HEADING_CATEGORIES', 'Genres');