int ledPin = 13;
int buttonPin = 2;
//volatile ISR = button_ISR;
int ledToggle;
int previousState = HIGH;
unsigned int previousPress;
volatile int buttonFlag;
int buttonDebounce = 20;

void button_ISR() {
  buttonFlag = 1;
}

void setup()  {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), button_ISR, CHANGE);
}

void loop() {
  if ((millis() - previousPress) > buttonDebounce && buttonFlag)  {
    previousPress = millis();
    if (digitalRead(buttonPin) == LOW && previousState == HIGH)  {
      ledToggle = ! ledToggle;
      digitalWrite(ledPin, ledToggle);
      previousState = LOW;
    } else if (digitalRead(buttonPin) == HIGH && previousState == LOW) {
      previousState = HIGH;
    }
    buttonFlag = 0;
  }
}