You can access the edit item page by hovering your mouse over the "Selling" link in the upper left corner of any page then selecting "Add or edit items" from the drop down.
Once you're on the "Add or edit items" page, simply click the "Edit" link on the tool menu that shows above each item.
Clicking the "Add a new Item" button will take you to a blank New Item Form, or "edit item" page, to put in your product information, like this one:
Looking to edit multiple listings at once? Check out our handy batch editor.