Adjusting the bicycle handlebar height

First, you need to loosen the handlebar from the fork. Look on the top of the stem, where it links with the bicycle frame. On the top, you will notice yet an other hexagonal screw. If you do not see one, look for a small round plastic cap. Remove that cap with the flat screwdriver or gently using a knife blade (be careful not to pull the blade toward you!). Okay so you located the hexagonal screw. Loosen it until the head of the screw is a good centimeter out of the stem. The screw now looks totally loose but the handlebar is still tightly coupled to the fork, why? Well, there is a special mechanical piece at the end of that screw that acts like a lock. It is made of a wedge or an expander nut. To release the grip, lightly hit the top of the screw with a hammer. Do not hit it hard. Start lighter and augment the hammer force until the head of the screw goes back in the stem hole. Once that happens, the handlebar should freely move in all directions.

Image source: (Bicycle only). Modified by The bicycle Shopping Guide.

The procedure we will learn here is valid for almost all types of handlebar stems. If your bicycle uses a "Threadless" type stem, you will have to work harder to change your handlebar height. With these stems, the handlebar do not lower directly inside the fork. Instead, you may have to remove one or more rings around the tube attaching the stem to the fork. For some models, you may even have to cut a few millimeters of that tube (the same width as the rings you removed basically). As you probably figured by now, in this case, this is a no way back process. Once you have lowered the handlebar, you will not be able to raise it up again. Make sure you need to lower the handlebar before proceeding. If unsure, consult a bicycle specialist.