Button to update order status Woocommerce with WC Marketplace

Question

Thank you all, I did found the answer HERE.

I adapt a bit the code to my needs and added a button.

html:

<div>
  <form method="post" name="update_status">             
    <button type="submit" name="marked_as_completed">Mark as Completed</button>
  </form>
</div>

php:

<?php 
        if (isset($_POST["marked_as_completed"])) 
        {
        $completed_status = $order->get_id();
        $order = new WC_Order($completed_status);
        $order->update_status('completed', 'order_note');
        echo '<h2 style="color:green;">Completato &check;</h2>';  
        } 
?>