#define LED 13
#define BTN 4

byte lastButtonState = LOW;
byte ledState = LOW;

unsigned long debounceDuration = 50; // millis
unsigned long lastTimeButtonStateChanged = 0;

void setup() {
  // put your setup code here, to run once:
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);

}

void loop() {
  // put your main code here, to run repeatedly:
  if (millis() - lastTimeButtonStateChanged > debounceDuration) {
    byte buttonState = digitalRead(BTN);
    if (buttonState != lastButtonState) {
      lastTimeButtonStateChanged = millis();
      lastButtonState = buttonState;
      if (buttonState == LOW) {
        ledState = (ledState == HIGH) ? LOW: HIGH;
        digitalWrite(LED, ledState);
      }
    }
  }
}