Look around it, see if you can find a grub screw on the flash hider. Loosen that (Most likely with an allen key) and you should be able to twist it off.
If that doesn't work. Make sure you're not twisting the wrong way. Some threads have + or - threads.
Aaaaand if that doesn't work. Check the thread direction on it (Google your gun and see if you can find its thread direction) and just grab it as firm as you can and muscle it off I guess.