As you can easily see in the pictures, and as [MENTION=185012]Joe Remi[/MENTION] and [MENTION=98328]zebede[/MENTION] state in posts above, the large hex bolt on the lower end needs to be loosened. Go carefully, because you don't want to accidently drop a nut down the fork steering tube and for some reason (say corrosion) you are unable to fully remove the stem, retrieve the nut, and reinstall it.

Loosen until the bolt can be turned with your hand. Then tap the top lightly with a hammer. Holding the wheel between your legs, see if the stem/bars will pivot while the wheel remains in one place. If it does, align the steering and retighten. If it doesn't try soaking everything in penetrating oil (WD40, etc.) and try again in a day or two.