Why on earth would you bid if you don't want it?
Sigh.
Some peoples' children.
If you are just interested in seeing how the auction goes, there is a Watch This Item button just below the Bid Now button.
If you have bid and changed your mind, you can Retract your bid. Go to the Site Map at the bottom of this page. The Retract is near the bottom of the left hand column. Reasons are few.
Or you can contact the seller and ask him to cancel your bid. Be aware that he may also Block any future bids from you.