Here is my canned response on Lowering an existing Proxy or Max Bid. If you need more information, please let me know, giving details about the problem at hand.

Once a bid is entered into the system, it cannot be lowered. It can only be raised.

The reason for this is to drill into our heads that every single bid we make (and I mean every single solitary last bid) is extremely serious and is a legally binding contract and agreement to pay, and to pay the full amount if necessary.

The only way to effectively lower your Proxy Bid is to retract and bid again. This carries its own set of problems, and eBay recognizes only a few valid reasons to retract. You definitely don't want to get into the habit of retracting.

Unless the full amount of your proxy bid will absolutely break your bank, I would advise to hang tough. Maybe your proxy won't get nibbled up, or maybe somebody will outbid you and win. At the worst, you'll spend a few dollars and learn a valuable lesson.
by Tyler Jones