Once an order is placed we're unable to change any of the details. Any required changes would need to be done by placing separate orders.
If you are looking to add a further item to your order, we recommend that you add it onto a future order, doing so will save on the postage costs which are charged for each new order.
If you are wanting to cancel an item from an order you've already placed, we're unable to stop the order before it is shipped. Please check the How do I return my item?article to understand how to return an item once it's been received.