To earn a Boeing 737 type rating certificate, pilots must complete 21 or more days of instructor-led academics and simulator training. To be certified to fly a 737 MAX, a pilots must either complete a 737 MAX specific type-rating course or, if a pilot is already certified to fly the 737NG, they must complete the NG to MAX Differences training.
https://www.boeing.com/commercial/737max/737-max-software-updates.page

