So the trick is NOT to follow the instructions. Leave the monitor ON the stand, do not remove. Push down on the top of the monitor to make it go down.

I see that my instructions were confusing users. So I made changes to clarify. The purpose to removing the LCD panel was simply to verify that the Stand riser vertical riding slot was functional.

On the Stand riser itself, see the vertical riding slot below the square mounting bracket.

That vertical riding slot shows the allowable up/down movement of the connected LCD panel. The LCD panel is attached to the Stand riser. If it is already all the way up, you can only go down. If it is already all the way down, you can only go up. There are no buttons. You simply grasp the top and bottom bezels of the LCD panel and push or pull up and down.

Yes, and what you do not understand about the problem is the return spring forces the monitor to the topmost  position.   They are being shipped now and the weight of the monitor is insufficient to leave the monitor at an adjustable position. What the original questioner is asking is how to lower the return spring pressure or mechanically lock to a position.  I am asking because I now have 60 monitors (30 mad customers) who cannot set the height.  Until dell get the documentation and the monitor fixed I am going to write negative reviews on this series of monitors,  Listen Chris, 60 out of 60 do not lock. Poor qc if the manufacturer changed the spring tension without notifying dell.   

@Waternwaves​ 

I also have the same issue. Please help what should i do it is not locking.

Similar problem here: the height of the stand riser cannot be changed at all - it's fixed firmly in the topmost position, as if it were locked. Even applying force doesn't help. Any solutions?