Fixed bids, transmitted to auction houses for saleroom sales, can be changed or cancelled before the sale. However, please contact the auction house to confirm that your request to change or cancel your bid has been taken into account.
Automatic bids can be changed or cancelled before or during the sale, unless your bid is the decisive bid in the auction.
Simply click "My Account", then "Bids & Absentee bids" to view, change or cancel your bids. This is also possible directly from the lots' pages.