By placing a bid on an auction you are entering a legal binding contract therefore you cannot remove your bid.
Be very cautious when placing bids on the website. There are safeguards in place when bidding to prevent mistakes from happening. Two buttons need to be clicked before the bid is sent, make sure you triple check the Auction Details page to confirm that is an auction you really want to bid on.
If you want to reduce your bid amount, you can.
My account->Auction Tools->Bid Manager->Update Max Bid->Enter Max Bid.