If you need to cancel an order you can do this from your order page of the order. To find this, click on the 'Dashboard' tab on the left-hand side of the screen, find the order you want and click on it to open it.
If you scroll right to the bottom right of your order page you should see a cancel button which will cancel and refund your order when you click it.
If you cannot see the cancel button that means that cancellation is no longer possible.
Note: You can cancel an order free of charge directly from your account up until the day of pickup.