Once placed, there is no way for a seller to decrease a buyer's bid. All you can do is cancel it in full. If you do, it's recommended you add the buyer's ID to your Blocked Bidders List so he can't disrupt your auction further.
On the reverse side, the bidder can retract his bid... although, this could be against eBay's policies, depending on the exact situation. If he made a serious typo (entered $100 instead of $1), then allow him the benefit of the doubt as long as he re-enters a corrected amount.
Keep an eye on your listing. If you decide you don't want to deal with this bidder, you always have the option to cancel his bids & add him to your BBL.
Good luck!
Graciously Grin When You Growl