Clicking on the Cart Button will open the Cart Panel (Mini Cart).
The Cart Button is a State Button. This means that text and images can be dragged and dropped into it. It is fully customizable, you can change the fill, stroke, font, add images, etc.
Customizing Cart Button Further
You can customize the Cart Button by deleting everything inside of the Notification state button and then drag images and text inside of it.
You can also choose to use your own objects as the Cart Button. This can be done using graphic styles. An article explaining how to do that can be read here.