I had to change something on a short time-frame on items with bids.I could only use the bulk editing tool thing which will add a note at the bottom of the listing.Well if people read things it's ok.

As Helen suggests you can end the listing with bids if it is i think 24hr before ending if auction.I have read that eBay may charge you for ending early with bids though.I guess it depends how much $ is on the line with the mistake.

Good luck.