The handlebars can be adjusted to any height within the range of the lowest and highest setting (with the black button in the locking hole.)
To do this, all you need to do is open the clamp that sits in the middle of the T-bar, lift the handlebar to the desired position and close the clamp again. This should lock the handlebars in the desired position.
If the handlebars fall down when your child is riding, then it would indicate that the clamp is not tight enough. It order to tighten it, you'll need to open the clamp and using a 5mm Allen key, turn the bolt here clockwise. By tightening this bolt, we’re increasing the amount of pressure that the clamp can apply.
Ideally, when you go to close the lever it will hold some tension - but can still be opened and closed with relative ease. If there's no tension - it's too loose. If you can't close the lever fully - it's too tight.
If you’ve followed the above steps and the handlebar is still falling down, then it would pay to double check that the black plastic spacer that sits between the bottom tube and upper handlebar extension is still there. You can compare your own scooter to the picture below: