You have to remove the upper first.
Drop mag and check chamber...
When clear, remove dust cover, then recoil spring, then bolt carrier assembly.
On the right side of the rear sight block there is a little lever, turn it to where it points straight up.
Pull the rear of the upper handguard/gas tube straight up, if it doesn't move, turn the lever from above slightly to line up the flat spot on the cross pin until there is clearance for the gas tube to come off.
Once it's off, look at the metal retainer on the front of the lower handguard. On the right side there is a small lever tucked in on the side. Using a screw driver, rotate it up and 180 degrees. Now you should be able to slide the retainer forward. Tap it with a mallet if needed.
You should now be able to remove the lower handguard by pushing it forward and rotating it out.