You can just edit the listing from Saved Listings (here at Auctiva) and then post.
But if you want to get your relist credit from eBay then edit it here, save it but don't post it. After you've saved it go to Saved Listings, tick the box next to the listing and then hit the "Get HTML" button at the top.
Then go relist through eBay and plug that HTML code into the description box.
Hope that helps.