In these circumstances it's better to cancel the transaction (the refund will be issued in the process) and relist nearer the time for the buyer to buy and pay again.
Tell buyer what you're doing first and once they agree click on the dropdown menu arrow alongside the item on your orders page and select Cancel order > Buyer asked to cancel.
If they're difficult about it you can cancel as above but use the reason "Something wrong with buyer's address" and put their ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock
and relist the item if you want to.