How Do I Update A User?

[gb1200]

How do I update a user using the data manager user?

I'd like to update different fields at different times such as username or email

I've got this but it doesn't seem to work.

$userdm =datamanager_init('User', $vbulletin, ERRTYPE_ARRAY);
$userdm->set_existing($vbulletin->userinfo);
$userdm->update_username($userid, $username_new);
$userdm->save();

Thanks