1) Click on 'Manage Inventory' under the 'Inventory' heading in 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) On this page you can edit all of an individual item's properties.
5) When you are finished editing your item, be sure to click the 'Save Changes' button to finalize.