How to use update_post_meta with Metabox?

I'm trying to assign an order's product SKU to a custom value, like this:

add_action( 'woocommerce_thankyou', 'change_value' );
 
function change_value( $order_id ) {
   $item_sku = '';
   $order = wc_get_order( $order_id );
   foreach ($order->get_items() as $item) {
     $product = wc_get_product($item->get_product_id());
     $item_sku = $product->get_sku();
   }
   update_post_meta( $order_id, 'custom_value', $item_sku );   
}

However, I can't make it work and I can't find anything about how to update a custom value from Metabox in a similar way to this. Can you help me? Thanks!