To modify an existing template as Brent mentions above, just go to the Listing tab, then click on "Manage" in the Custom Templates area, and then click on "Create from Existing." this will open an area at the bottom of the page with a list of all our existing templates. Find the one you want to modify and click on it, then click the "Create" button next to the list. You can then modify the HTML and save it as a custom template! Hope that helps.
-- Auctiva David