how to change the web.config after the deployment auto matically

<defaultProxy enabled="true">
      <proxy autoDetect="True" scriptLocation="http:some value" />
    </defaultProxy>
  </system.net>

should be changed to like below

<system.net>
    <defaultProxy enabled="true">
<proxy proxyaddress="http:some ip value" >
 </defaultProxy>
  </