If your bid is an autobid... it can be amended or deleted on the lot page up until the auction starts. If the auction has already started, you can amended or delete the bid in the live bidding applet.
If you've left a max bid before a live auction starts... Unfortunately, a this type of bid cannot be decreased or removed before the auction starts. Once the auction has gone live, you can amend or delete the bid in the live bidding applet.
If your bid is a live bid... if you've accidentally clicked the bid button during a live auction you will need to contact the auction house as soon as possible to explain what has happened as the cancellation of the sale is at the discretion of the auctioneer.