Yes. If your maximum bid amount has been outbid, or you want to increase the dollar amount of your existing winning bid, you can place another bid at a higher amount. To increase your current bid amount, simply return to the listing and submit a new, higher maximum bid amount (this amount is the most you’re willing to pay for the item).
You won’t be bidding against yourself when you want to increase your maximum bid amount, even if you’re the current high bidder. You only have one maximum bid amount per item, so you are simply increasing it.