The quickest way to cancel items from your order is to do it yourself, by signing into Your Account and visiting the Despatch centre. At the Despatch centre you will see all your open orders and an indication if it has been despatched to you. You can cancel part of the whole order, only if the order hasn't been despatched to you.
To cancel part of the whole order, find the order you want to change and go to the order details page. Click "Manage order" located on the top of the order details screen, select "Cancel part or whole order" and press the "Next step" button. On the following screen tick the box next to the item you want to cancel and press the "Submit" button. To cancel the whole order, select all items and press the "Submit" button.