Changing Polarities on DFRobot 2A Motorshield

How to change to polarties of the motor shield to change the direction of the shaft? DFRobot's 2A motor shield has only 1 IN ang 1 EN for each motor.

From what I've learned, to be able to switch directions, you need to change the polarities. Is there something I am missing?

Also, the DFRobot Motor Shield Twin's wiki includes a code in which it says something about changing the speed and direction.