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.