<?php
namespace VendorModuleControllerOrders;
class SetOrders extends MagentoFrameworkAppActionAction
{
protected $orderItemRepo;
protected $resultPageFactory;
public function __construct(
    MagentoSalesApiOrderItemRepositoryInterface $orderItem,
    MagentoFrameworkAppActionContext $context,
    MagentoFrameworkViewResultPageFactory $resultPageFactory
) {
    $this->orderItemRepo = $orderItem;
    $this->resultPageFactory = $resultPageFactory;
    parent::__construct($context);
}
public function execute()
{
    try {
        $orderId = 6;
        $order = $this->orderItemRepo->get($orderId);
        $order->setAdditionalData('123123100');
        $this->orderItemRepo->save($order);
        echo "success";
    } catch (Exception $e) {
        error_log($e->getMessage());
    }
}}