Do you really want to change the status of the product or the product variation?
$items is an array of OrderItems. If you loop over the items, you can get the referenced purchased entity of the single item with
$purchasedEntity = $item->getPurchasedEntity();. If you only want the first item you can access it like so
$purchasedEntity = $items->getPurchasedEntity();.
In case the PurchasedEntity is a ProductVariation you can get the product with
$product = $purchasedEntity->getProduct();
In case you want to change the status of the product:
Or for the variation:
If you want to set any custom field on the product variation:
This assumes you use the default commerce2 product architecture. It is absolutly possible that you get other purchasable entities than ProductVariations back from the OrderItem.