Modify product
The modify product page contains all of the original "Add new product" options that you can edit and also provides a few additional more advanced options such as the following:

Product options
Bulk pricing
Upselling links
Detailed images

Click on each option above for a detailed description.

You can access the "Modify product" page by clicking the left side "Modify product" link under "Products".
 

Or you can click the "Modify product" icon on the main page of your personal administration area.

Once clicked, you will use the "Search products" interface to chose the product that you wish to modify.