WC_Order::update_status PHP Method

    public function update_status($new_status, $note = '', $manual = false)
    {
        try {
            if (!$this->get_id()) {
                return false;
            }
            $this->set_status($new_status, $note, $manual);
            $this->save();
        } catch (Exception $e) {
            return false;
        }
        return true;
    }

 /**
  * Update a order status
  *
  * @return void
  */
 function change_order_status()
 {
     check_ajax_referer('dokan_change_status');
     $order_id = intval($_POST['order_id']);
     $order_status = $_POST['order_status'];
     $order = new WC_Order($order_id);
     $order->update_status($order_status);
     $statuses = wc_get_order_statuses();
     $status_label = isset($statuses[$order_status]) ? $statuses[$order_status] : $order_status;
     $status_class = dokan_get_order_status_class($order_status);
     echo '<label class="dokan-label dokan-label-' . $status_class . '">' . $status_label . '</label>';
     exit;
 }