Oh, to get to the hopup, pulldown the dust cover. Pull the charging handle back, it should open it up. If not, then manually pull it open. Hop up dial will be right in there.
Edit: You posted before I did.
See if the flash hider has a screw in it. sometimes they lock them down using a screw. If so, unscrew it, and then turn the flash hider accordingly (Righty tighty, lefty loosey, or reverse that for CCW threading).
If it's epoxied on, do what I said to do.