when you created the cart display using the display manager, make sure to select to use an updateable display type on the first page, that will add an updatable quantity field.
the cart display is not a single server behavior that can be updated. the display manager is a collection of behaviors that get added based on your selections in the display manager.
If you want to make changes, you will need to make the changes manually.
for example, if you want to add a cart column to the display, you will edit the table to add a new column, then use the bindings panel to add the cart column binding.