How do you change a fan belt on a 72 bug?

You should get Muirs book and or the Bentley manual as they have a HUGE amount of info. They have a two piece pulley system. Put a medium size straight screw driver in the slot of the inner Generator pulley and then it will hold the pulley from moving when you take off the nut. Keep track of the "shims". Put "shims" between the two pulleys or between the nut and outer pulley to tighten or loosen the belt. Once you get it right put the remaining "shims" between the outer pulley and the nut and then torque it down.