How do you remove a starter on 21 hp Briggs and stratton?

It depends on the engine. The single cylinder and V-twins will have two bolts at the top of a flange on the top of the starter that must be removed after removing the electrical connection. Often shrouds and/or a cover needs to be removed first to gain access. Usually a 1/2" socket will do and some have a place for a T-30 torx bit as well.

On an opposed twin it is a bit trickier since the bolts are in a vertical attitude and it appears that the flywheel must come off. This is not the case since there are recesses in a couple of places on the underside of the starter that will allow the bolts to move upwards into them during removal.