Yes, you can still change an order after it has been made. You can change the (amount of) products the order consists of and the address that is linked to the order.
You can change the (amount of) products the order consists of in your Dashboard. Simply click on the order and cancel the order lines of the product you don’t want to send.
When you click on cancel, you can specify how many of the products you want to cancel.
As far as the details of the order are concerned, you can only change the address. You can do this via the API. Read here how to change the address in the order.