#define BUTPIN 8
#define LEDPIN 4  //or use LED_BUILTIN

void setup() {
  // put your setup code here, to run once:
  pinMode(BUTPIN, INPUT_PULLUP);
  pinMode(LEDPIN, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(LEDPIN, !digitalRead(BUTPIN));
}