Try pulling the stem in and out. Look for anything moving whilst doing this. I am going to guess that a lever will poke out the side of the movement somewhere near the stem going in. Then you will push the lever down.
Also look along the "sides" of the movement. sometimes a lever is recessed in there, and can't be seen from directly above.
Just look for movement, and where you see that, you will find your button.
Could also be the screw above the one adjacent to the stem going in. The closest one holds the cover down by the look of the indent, so you could try that other one, Don't unscrew it more than 2 turns.
Let me know if that doesn't work and we will investigate further.
O/S