1) Click on "Manage Inventory" under the 'Inventory' heading of the Control Panel.
2) Search for your item by it's Item ID (or Author / Title)
3) Scroll down to the 'Search Results' and click the 'Edit' link under your item.
4) Click on the 'eBay Properties' button.
5) Scroll down to the 'Shipping' heading & change the 'Package Type'.
6) When you are finished, be sure to click the 'Save Properties' button to finalize.