Hold the stem so the teeth are facing you. Loosen the screw on your left side 1 1/2 turns. Now loosen the screw facing you (above the teeth). This screw holds a small plate, which engages the teeth to lock the stem. You may have to gently lift the plate off a tooth the first time, once you loosen the screw.
Adjust the angle as desired, tighten the screw above the teeth first. Then tighten the screw on the left side. I believe this is the same stem that Level uses on the 500, although they may have used a different size.
Note that you have to install it before you can set the angle, as it needs to be at about 10 deg to be able to install it (screw on top). This is a solid unit, and the adjustment is mechanically secured, not a friction hold.