Grab ahold of the top and give it a push (or pull). If it's not too tight, you can "wrestle with it" to adjust it. Technically, you're prolly 'poseda loosen the bolts that hold it, adjust, and re-tighten. In terms of height, I'm sure there's a recommendation... someone will chime in with how far in front of the bike the light is supposed to be "on" the pavement.