Simple Delivery is not a flexible system. To keep it official you should really cancel/refund with buyer's agreement (via the dropdown menu for the item on your orders page), correct your address on here:
https://accountsettings.ebay.co.uk/uas/addresses
and the relist for the buyer to buy again.
The alternative would be just to write your correct address on the parcel. It depends on the value of the item. If it's a few pounds I would risk a written correction. If it's expensive I'd cancel, correct and relist.