Form you main menu chose Catalog > Edit Catalog & Categories

Navigate to the product you wish to edit and click the edit icon:

Once in the product editor select the Descriptions tab:

From here click the setting tab:

From here you can change the labels of the tabs that are displayed for this specific product by typing into box to the right of label. While there are setting to override these value for a specific theme or to override these labels for the whole, making this theme specific change at the product level is not recommended in order to make theme level changes to these label please read article: http://www.americommerce.com/kb/?f=451  as it is much easier to manage these changes from the location mentioned in article # 451.