If you are talking about doing it from your "my ebay", all you have to do it click the drop down box to the far right of the item - there should be an option to relist. Go in and change the selling format to "store", then choose the duration.
If you are talking about doing it from Auctiva, you would just pull up your saved listing, click the button at the top that says "store", choose the duration, and launch.
Hope this helps.