Such a great question- There is no Help Topic on orders to deal with this question. The Cancel option is available to Sellers in the More Actions tab on an order. Perhaps, message the store and explain your wish to edit your order.
I had a customer cancel an order the other day. On doing so it returns the items to stock, so theoretically you could ask them to cancel, then reorder using an alternate payment method straight away (assuming no-one else has bought them in that split second!)
Comments
The Cancel option is available to Sellers in the More Actions tab on an order.
Perhaps, message the store and explain your wish to edit your order.