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.
The circular dial in the top middle of your picture adjusts the tension. Some of these require you to push it in to allow movement, others just need more force to turn. Once you have the helmet on your head you reach back and twist the dial FCE one direction to loosen and the other to tighten. It is the same as a welding mask and I prefer to actually hold the helmet very slightly off my head in the middle so it floats using the headband, allowing more air flow. It is much cooler and also the weight is more even around your head rather than rubbing in one place the whole time.
Make sure as well that you bought the right size helmet for your head size. Ideally, it should stay in place even without the adjustment dial at the back (that just makes it even more secure). And you should have around a two finger distance between your eyebrows, and the bottom of the helmet.
Yea, we figured out you have to angle it slightly inward or ourtward when pulling or pushing from the bottom. Once you know, it's super easy, but until you realize that it's basically glued in place.