Transformation point

This seems like the dumbest question ever, but does anyone know how to move a transformation point on a movie clip? I have a gun turret that I want to rotate around a point at the base of the gun and all I want to do is move the transformation point. Everything I read says all I have to do is click it and drag it, but there’s nothing I can do to select it; if I try to drag it, it simply drags the entire movie clip.