To remove the freehub body, first remove the axle. Insert a 10 mm Allen wrench into the fixing bolt in the middle of the body. Unscrew the hollow fixing bolt (conventional right thread, counterclockwise to loosen). The body will lift off, it is secured to the hub shell by the hollow bolt and a set of splines that keep it from rotating.
Dura-Ace bodies require a special spline wrench, Shimano tool #TL-FH10 to remove the body from the hub. See my page on Dura-Ace Interchangeability for more details on this.
You might need a long one to get some sufficient torque on it.