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) On this page you can edit an individual item's eBay properties. You can update your Pricing, Best Offer, Category & Listing properties. from eBay. As you scroll down you can edit your item's Shipping, Return & Payment Policies as well as your Package, Stock & 'Quantity Per Buyer' properties.
6) When you are finished editing your item's eBay properties, be sure to click the 'Save Properties' button to finalize.