rotate.cs

int nextY = 0;
int direction = 1;
private void Update()
{
    if (nextY == 90 || nextY == 0)
        direction *= -1;

    nextY += direction;
    var nextRotation = new Vector3(transform.rotation.eulerAngles.x, nextY, transform.rotation.eulerAngles.z);
    transform.rotation = Quaternion.Euler(nextRotation);
}