Unfortunately, it is not possible to change your address after the transaction has entered the "Order completed" stage. In order to change your address, please access the following page and cancel your order first: Menu > My Page > Shipping.
Orders cannot be canceled during the "Preparing shipping" or the "Shipping" stages, and customers who would like to cancel need to contact Customer Service (My Page > Settings > Support > 1:1 Inquiry). More information can be found in the "How do I return/refund a purchase?" article in the FAQ.