Hardware Interrupts

Projects General Guidance
3 
const uint8_t buttonPin = 2; // a momentary push button connected to pin 2 on the Uno
void setup() {
    pinMode(buttonPin, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(buttonPin), upward, LOW);
}
void loop() {
    if (digitalRead (buttonInt) == LOW) {  // This is completely unnecessary, because you already check the LOW button state using an interrupt.
        digitalWrite(forwards, HIGH);
    }
}