There is a trick to unlocking it:

First, push the screen straight down with one hand (it will move down a few millimeters or so from its current position).

While holding the screen down, push the rectangular button that is on the back of the stand. You will notice that the button pushes all the way in now that you are holding the screen at its lowest position.

Then, while holding that button down with one hand, raise the screen with the other. You should now be able to raise and lower your monitor screen freely.

The trick is to push the monitor screen all the way to the bottom (until it goes no further) before pushing the button.

Hope this helps.