updating via cmis sync.

I have defined an alfresco content type in Drupal in settings.php (the two $conf entries that I found in the documentation). I have added a form, and without adding further code I can add content to Alfresco from the drupal "create content" page. If the content already exists, the form is prefilled with the existing content that I get from Alfresco through the cmis_api. However, when I press "save" on the "create content" form when the document already exists, I get an error from Alfresco saying that the document already exists. What can I do to make updating a document as easy as creating one?