If you can not make a delivery contact the buyer directly as soon as you can and try to arrange for an alternative delivery date. If you are unable to work out an alternative delivery, cancel the order.
To cancel an order find it on the Orders link in the navigation header and click view order. Towards the bottom, click on the red cancel order button. You'll be able to enter a reason for the cancellation, which will be emailed to the buyer, and choose whether or not to put this product back in stock.