<sci_request version="1.0"> 
  <send_message> 
    <targets>
      <device id="00000000-00000000-00409DFF-FF(place_gateway_ID_here)"/>
    </targets> 
    <rci_request version="1.1"> 
      <set_setting>
        <simple_password>
         <password_mode>test_password</password_mode>
         <username>root</username>
         <password>replace-this-password</password>
         <public_comm>public</public_comm>
         <private_comm>private</private_comm>
         <public_key/>
       </simple_password>
     </set_setting>
   </rci_request>
  </send_message>
</sci_request>

You may be prompted for your Remote Manager account credentials.