This is a pic of the adjustable stem of my 2020 Trek Verve 3.

To adjust the angle of the stem, what do I do?

I tried loosening the bolt to which the yellow arrow point: no joy. I could not change the angle.

I then found the bolt to which the red arrow points, but was unable to budge it. It was so tight that I stopped for fear of stripping it.

Anyone know what's the trick? Thanks if you can help.