title: Paymill API: How do I update a subscription's credit card?
body: <p>I can create subscriptions, but I can't figure out how to update a subscription so that it uses a new credit card.</p>
  
  <p>This is what I've tried: (assuming there is already an existing client, payment and subscription)</p>
  
  <ol>
  <li>create token in ui</li>
  <li>create a payment object in server</li>
  <li><p>update client like this: </p>
  
  <p><code>paymill.clients.update(clientId, { payment: payment.id }, callback);</code></p></li>
  </ol>
  
  <p>I get this error:</p>
  
  <pre><code>Error: Identifier,payment
  Exception: Api_Exception_InvalidParameter
  </code></pre>
  
  <p>Which calls do I need to make to achieve this?</p>