Locate the main bolt for the handlebar, which runs vertically downward from the top of the fork. The bolt runs down through the stem of the handlebar and stops inside the fork. This is the bolt to loosen in order to be able to adjust the handlebars.
Insert the Allen wrench (usually a 6 mm Allen wrench is the standard size but try the next size up or down if the 6 mm wrench doesn't work as sizes may be slightly different) into the bolt and turn the wrench to the left, or counterclockwise. A few full rotational turns on the bolt should be enough to loosen it sufficiently.
Adjust the handlebars while facing the bike from the front with the tires in between your knees. This positioning will give you the best leverage and control for adjustment. Squeeze your knees tightly on the bike tire while you pull in an upwards motion on the handlebars. As you move the handlebars up, you should notice a maximum height marker on the handlebars. Be sure not to raise the handlebars above this point as this will be a danger to the rider of the bike.
When the handlebars have reached the desired height, tighten the bolt. Tighten the bars to the point where the bars can no longer move up or down when you apply all your weight to try and move them.