As a rule Ebay does not allow buyers to cancel their bids, in order to protect the sellers from incurring fees without actually making a sale. Every bid is considered a contract and you are obligated to buy the item, unless it is listed under a “non-binding bid policy” category. In case of an honest mistake however, you can request a cancellation if the conditions and time restrictions apply.
Please note that once you retract a bid, it can no longer be reinstated.