You can edit an existing product by selecting More from the top menu and clicking on Products from the dropdown, 

Find the product that you want to edit and click on the Edit button next to that product

Modify the information in the pop-up box for that product

Editing a product will also modify the product details in all the leads where the product is attached. For example if the product price is updated then the value of all the leads associated with the product will also be updated accordingly.