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 'Pricing' tab.
5) Next to 'Quantity:' edit your item's quantity.
6) When you are finished editing your item's quantity, scroll down and click the 'Save Changes' button to finalize.