public function store(Request $request)
{
    // ...

    // iterate (loop) over each item in the order items collection
    $order->items->each(function ($item) {
        // deduct the quantity of the item for this order
        // from the stock of the product and update the new stock
        $item->update(['stock' => ($item->stock - $item->pivot->quantity)]);
    });

    // ...
}