Be sure that the lock is not engaged. Grasp the lens firmly on a portion that is fixed (does not turn). Grasp the camera in the other had. Firmly twist the lens in the "off" direction (usually counterclockwise) increasing pressure gradually.
It may also be possible to loosen the lens by holding an air can upside down and squirting the liquid onto the area that seems to be binding the most. If you do this, use only a short squirt, and don't repeat it. The spray will cool the metal and may cause it to contract enough to loosen, but cooling things too much may cause condensation inside the lens. This is the sort of thing techs do but don't tell customers about.
If you try the air can DO NOT GET THE LIQUID ON YOUR SKIN OR IN YOUR EYES. It is extremely cold, and will cause frostbite.