title: Change shipping method programmatically from existing WooCommerce order
body: <p>My customers ar buying subscriptions through my woocommerce website. They receive the products each month but somethimes they want to change the shipping method.
  I don't find doc for doing it through php.</p>
  
  <p>I could change the values in <code>post_meta</code>, <code>woocommerce_order_items</code> and <code>woocommerce_order_itemmeta</code> but it's not a durable solution.</p>