https://medium.com/jungletronics/adafruit-motor-shield-v1-v2-2fa1430edaf9

_54_Adafruit_v2_04.ino

/* Adafruit v2 */

#include <Wire.h>
#include <Adafruit_MotorShield.h>
#include "utility/Adafruit_MS_PWMServoDriver.h"

Adafruit_MotorShield AFMS = Adafruit_MotorShield();

Adafruit_DCMotor *frontMotor = AFMS.getMotor(3);
Adafruit_DCMotor *backMotor = AFMS.getMotor(4);

void setup()
{
  AFMS.begin();                     // Initilize Adafruit Motor Shield
}


void loop()
{

  frontMotor->setSpeed(150);        // Set half Speed
  frontMotor->run(FORWARD);         // Clockwise
  backMotor->setSpeed(150);         // repeat for
  backMotor->run(FORWARD);          // back motor
  delay(2000);                      // Wait 2 seconds

  frontMotor->setSpeed(150);        // Set half Speed
  frontMotor->run(BACKWARD);        // Anti-Clockwise
  backMotor->setSpeed(150);         // repeat for
  backMotor->run(BACKWARD);         // back motor
  delay(2000);                      // Wait 2 seconds

  frontMotor->run(RELEASE);         // Stop front motor
  backMotor->run(RELEASE);          // Stop front motor
  delay(500);                       // Wait half second
}