byte ledPin[] = {3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
int ledDelay(500);
int direction = 1;
int currentLED = 0;
unsigned long changeTime;

void setup() {
  for(int x=0; x<10; x++) {
    pinMode(ledPin[x], OUTPUT);
  }
  changeTime = millis();
}

void loop() {
  if((millis() - changeTime) > ledDelay) {
    changeLED();
    changeTime = millis();
  }
}

void changeLED() {
  for(int x=0; x<10; x++) {
    digitalWrite(ledPin[x], LOW);
  }
  digitalWrite(ledPin[currentLED], HIGH);
  currentLED += direction;
  if (currentLED == 9) { direction = -1; }
  if (currentLED == 0) { direction = 1; }
}