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 'Amazon Properties' button.
5) Scroll down to 'Publisher' under the 'Listing' heading. Fill out your item's Publisher.
6) Scroll down and click the 'Save Properties' button to finalize.