How to update WooCommerce order item quantity (existing order)?

Question

How can I correctly update/change/modify order item quantity of order items in existing WooCommerce orders?

(I am not talking about cart items before order creation like in How to update WooCommerce order item quantity )

I would please like to know both ways:

• via SQL ( UPDATE )
• via the WordPress way (wp functions)

I know that the product_qty field is located in the wc_order_product_lookup table.

I know that the meta_key field containing the _qty value ( where the meta_value field is containing the order item quantity) is located in the woocommerce_order_itemmeta table.

As a side note:

I am using the WooCommerce Advanced Quantity plugin ( https://codecanyon.net/item/woocommerce-advanced-quantity/11861326/support ) which enables decimal/float quantities (which I use as weight) instead of integer ones.

wc_order_product_lookup table: product_qty field - integer quantities (decimals are cut off)

woocommerce_order_itemmeta table: meta_key = _qty, meta_value - decimal quantities

Thus, both fields (integer & decimal/float) should be correctly updated.

Any clue would be highly appreciated.

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I found 2 resources pointing in the right direction:

1. Update and save an order item in the order on Woocommerce 3

$order = wc_get_order($ordernum);
  foreach( $order->get_items() as $item_id => $item )
   {
    $item->set_quantity($qnty);
    $item->save();
   }
$order->save();

2. https://rudrastyh.com/woocommerce/order-items.html

wc_add_order_item_meta( $order_item_id, '_qty', 2, true ); // quantity