Yes. You can raise your Proxy Bid (maximum) amount and you can also lower it, but not below the current winning bid amount.

To increase your current bid amount, simply return to the listing and submit a new maximum bid amount (this amount is the most you're willing to pay for the item).

Because the system will recognize you when you place the additional bid, you won't be bidding against yourself when you want to increase your maximum bid amount, even though you're the current high bidder. Your new Proxy Bid will still work to win the item for you up to your maximum bid amount, at the lowest successful price, if you are the high bidder at auction close.