If you have a Smith & Wesson M&P in 40 S&W, it is quite easy to convert it into 9mm by simply swapping the barrel and getting some 9mm magazines.
I did this on a S&W M&P 40 full size by buying a factory S&W 4.25″ 9mm barrel. It just drops right in without any fitting or adjustment. The breach face on the 40 and 9 are close enough that it is not an issue.
If your M&P 40 is a compact, just pickup some 9mm compact magazines.
If your M&P 40 is a full size, try these factory Smith & Wesson M&P 9mm magazines, these hold 17 rounds of your favorite 9mm ammo.