You missed the big bolt, about 21MM or so head size that is below the thin red arrow at the bottom of your picture. It threads from the front toward rear, horizontally through the center pivot of the tensioner. This bolt must be loosened to adjust the tensioner. You must pay attention to the arrow on the triangular scale.

What are you trying to accomplish? Is the belt slipping or are you just trying to change the belt?

Good luck,