You'll have to swap out your old mainspring and the top and bottom caps. Simple to do. The easiest way is to put the old mainspring housing in a vise, push down on the plunger and remove the pin. Keep pressure on the mainspring and lift it out. Reverse the order for the new mainspring housing.
or, you could buy the whole assembly and have a spare, or an extra set of parts. No problem changing them out, I just like to change them around more than what a normal human being should. Seems like I never, ever, have enough internal parts for mainspring housings.