1. By simply visiting your plugs page, hitting the options menu (3 small dots) for the plug you wish to edit, and clicking on "Cart Settings" in the drop-down menu.
2. You can also access the plug settings option while viewing a specific plug page. Scroll down to find the settings option and click the edit button.
Both of the above options will take you to the plug's cart settings page where you can manage various settings and the design template.