The quickest way to receive your new items before they sell out in the size you need is to place a new order online and send back the original order. Once your return has been received they will process a refund.
OR
Use the returns form included with your order to request an exchange (or download a form here). Provided they have the item available, they will send it via standard post and the shipping is on them.
If you select Exchange on your Returns Form, they cannot guarantee that the size/style that you have requested will still be in stock by the time they receive and process your return.
If they do not have the item available they will issue a refund and notify you via email.