Hello 'sweetjellybean',
Fortunately for you, yours is one of the few valid reasons for cancelling a bid, - entering
the wrong amount. Retracting bids is considered very poor form.
The following link will tell you all about it, and there is a Bid Retraction form you can use:
http://pages.ebay.ca/help/buy/bid-retract.html
It is good to familiarize yourself with ebay's position on such things, and if you look at the
Bid History page on the listing, at the bottom it tells that you can retract your bid under
certain circumstances, offering you a Learn more link.
Once you retract, you simply re-enter the correct amount. Easy, easy, easy. 
