It is hard to change payment method once you complete the order.
But depending on the order status, you can cancel the order and re-order the item.
1) On request
If you want to change credit card or payment method, please cancel the order and re-order the item.
But if seller has already confirmed the order, it is hard to cancel the order without seller’s approval.
2) Delivering / Delivered order
If seller sent out the item, it is impossible to change the order information(shipping address, recipient) and refund the order.