Dumb question, but new to cycling, and the person at the store adjusted the height of the back via the tab. The Trek manual and video are absolute garbage, so we can't figure out how. Can anyone help?
The dial behind the round part at the top center *(of your image). It should click and give you different tightness. Also the straps that click under your chin will adjust the fit.
Additionally, adjusting the height isn't usually a thing in my experience, just the tightness of the inner layer and the chinstraps. The shell should fit on top of your head in a uniform way other than these two adjustments.