First, you'll need to sign in (if you haven't already done so) and go to My Account. Under your account Dashboard, you will find a list of items you have bid on called My Bids.
Look for the item that says You are Losing.
To place another bid on that item, just click on its link and you will be taken directly to its detail page, where you can place another bid. From there, it's the exact same process you would have used the first time you bid, except that you'll need to enter an amount greater than or equal to the next minimum required.
Then watch the on-screen prompts that will tell you if you may have been outbid by someone else's Max Bid, or if your bid was accepted and you're winning. If immediately outbid, you'll need to repeat the bidding process until you see the status, You are Winning.