Once ordered, you can't change the quantity, size, colour or payment method, as preparation begins immediately. You can, however, cancel one item or the entire order in the first 24 hours, or before the shipping company receives it: open “My orders”, pick the order and choose either “Cancel order” or, for a single item, the “Cancel” button beside it, then give a reason.
Card payments are refunded to the same card, which typically takes between 10 and 14 working days, varying by bank. No cancel button any more? Then the window has closed, and you'll need to return the item once it arrives.
Source: Brands For Less: Returns and cancellation