eBay hide away the bid retraction form a bit, because they don’t like people using it. You can find it by going here: http://cgi1.ebay.com/aw-cgi/eBayISAPI.dll?RetractBidShow.

Now all you need is the item number from your auction: this can be found on the item description page’s top right corner. If you can’t see it on the page, look in your browser’s title bar, and in any emails eBay have sent you about your bid on the item. Choose one of the three allowed reasons, click ‘retract bid’, and you’re done.