Just revise your listing - you can click a link in your Auctiva Listings view, but as that only routes you to eBay anyway, you might as well go to the listing in eBay, click the 'Revise your Item' link up the top, and make the necessary changes. That's always assuming you are able to make the changes, depending upon the Revision rules that eBay apply - best bet is go and try.
HTH.