<message_body>
<pm:set_password>newpass</pm:set_password>
</message_body>

A setPassword message is sent by a client application or another cell service to change the password of the user in the <message_body>. If the original password in the <message_body> is correct then the password will be changed. Only a user can change their own password. If an admin needs to reset someones password, it is best to use the set_user_web_service call.
The domain, username, and password are sent in the header of the message so it does not need to be sent separately in the <message_body>.