You can retract your bid under certain circumstances. Go to the help menu in Ebay (At the ebay home page, there is a 'help' tab on the right hand side of where the Buy, Sell, My Ebay tabs are at the top of the page). There is a list of the top questions towards the top of the help page, and one of them is about retracting bids. If this doesnt show up just do a search, or use the help a-z index.
I think they have made it harder to retract bids these days (need a good excuse), but i think as long as the auction isnt within the last few hours you should be right.
Good luck