You can edit the properties of the cart within the cart object through the eCart object panel interface. This is where you configure things like charges, discounts and what columns are in the cart.
The cart display manager is a create only process, once you have added a cart display to a page there is no way to update it within the interfaces. Instead you need to remove the cart display that you have on the page and add a new one to get the newest updated version of the cart to display. You would want to do this for any cart display that you have created before updating the columns, charges or discounts in your cart.