You can modify your bids at any time on the My Vendor Listings page.
Simply click View/edit under Advertising Settings:
You can essentially name your price, though there is a Minimum Click Price that ranges from $.25 - $.45 per click. You can see your current Max Bid Per Click, along with the Current Top Bid for that county. Make any desired changes and then click Save.
You should set your Max Bid Per Click to be what you are comfortable paying for a click. IMPORTANT: You will only pay $.05 more than the next closest bid to hold your position. If you bid $1.00 per click and the next highest Max Bid Per Click is $.50, you will pay $.55, not $1.00. You will often not be charged the rate you set as your Max Bid Per Click.
To receive email notification when you've been outbid, simply check this box at the top: