how do i withdraw an item once its been put up on ebay??

my parents have put 2 reading tickets on ebay, but ive already promised them to a mate, so i want to withdraw them, but its saying i cant do it because theres less than 12 hours to go and its already got a bid.

how do i contact the bidder to say its already been sold, and is there anyway i can cancel the auction before the time is up??

help please!!