public function __construct(
    MagentoSalesApiOrderRepositoryInterface $orderRepository,
    MagentoSalesModelResourceModelOrder $orderResourceModel
) {
    $this->orderResourceModel = $orderResourceModel;
    $this->orderRepository = $orderRepository;
}

public function update()
{
    $order = $this->orderRepository->get(1); // 1 is  order id
    $order->setCreatedAt($createdAt); // set time
    $this->orderRepository->save($order);

}