I have huffy canyon mountain series bike. It is an old one. Can I raise the handlebars on it? I saw videos on YouTube about it. But all the videos have new bicycles with allen key screws. My bike is old. It has old hexagonal screw at the handlebar is about 20 cm long. Please help.
Its a Quill stem by your description.
To raise it, back off the Quill Expander Bolt by a couple of turns. Do not undo it completely.
Then if the stem doesn't move, tap the top of the Quill Expander Bolt lightly. The Stem Wedge is probably stuck, and tapping the bolt helps loosen it.
Raise the quill stem, but look for the Minimum Insertion Line which will be a stamped pattern of |||||||||| all around the stem's body. That line should be fully inside the bike. If you can see that line, its too high.
If that doesn't give you enough height then you need a replacement quill stem that is taller, or a threadless adapter and a threadless stem.
You might get sufficient height change by rotating the handle bars a little and then adjusting angle of your brake/gear levers. Or a replacement handlebar with some more height might be a good fix.