You have to loosen the locking ring either by hand are with a rubber strap wrench, then remove completely either one are two set screws that locks the hand guard to the barrel nut, once that is done you should be able to spin the hand guard off.
I am not to sure what make of hand guard that is, so what I have told you may not help. If this info does not help I am sure someone else will chime in and add to what I have given you.