You can end your auction early by navigating to the My Auctions>Open Auctions screen in your My Auctions pages, then selecting the Close button next to your item's listing.  There is also a Close this auction early link on the actual auction listing, but this is only visible to the member who listed the auction and only when they are logged in. Clicking this link will work in the same way as clicking the Close button on your My Auctions page.
 

Important: If your item has already received bids, you are obligated to sell the item to the highest bidder at that amount unless you notify them in advance with the reason of the item listing closure and then you must contact the administrator to remove their bids prior to closing the auction.