When a buyer requests that you cancel an order, the action you take will depend on whether you've already shipped the order.
If you've already shipped the order, you should contact the buyer to explain the order has already been shipped and offer the option to return the item. Upon receipt of the return, you can issue a refund for the order. You can contact the buyer from your Order List.
Learn more about handling returns and refunds.
If you have not yet shipped the order, you can cancel the order through you Seller Account. For instructions, see How to Cancel an Order.