There is no lever on armalite type guns, it's a wheel. Pull back the charging handle and the dust cover will open on the right side of the gun as well as the fake bolt will open. The hopup wheel is inside the fake bolt. It's the large wheel of the three you want to adjust. Clockwise for more hopup, counterclockwise for less. Adjust until your bb's go as flat and as far as possible. Depending on the gun, it should be around 100'-150'. Be sure to make the adjustments when you can safely and legally discharge your gun and where you have enough space to make it effective, as in not indoors (unless you have access to an aircraft hangar.)
Gun removed for clarity.
And BTW, all armalite type hopups go out of whack and need to be readjusted. How often depends on a lot, but when you see your bb's not ranging the way they should, make an adjustment. You'd probably even have to adjust it if you change brands and weights of bb's.