Deleting the bid on a manually closed item (meaning the item was manually closed via item management or "record bid" and did not close based on date and time) will reopen the item. You will need to award/close the item via item management by selecting the item and clicking "close selected," or have the item close by time and date.
Additionally, you can record a manual bid under Items > Record Bids or Auction End > Record Bids and award the item to a new bidder.