How to programatically modify location Element in web config

Hi memebers,
  How i can programaticaly modify the location element in web.config.For example in my web.config i have one location element
 <location path="elmah.axd">
    <system.web>
      <authorization>
       </authorization>
    </system.web>
  </location>
</configuration>
 
i would like to know how i can programaticaly(from code behind at run time) change the authorization under this location element.On an event i want to open the web.config and add new users to authorization.i would like to get out put like this
 
<location path="elmah.axd">
    <system.web>
      <authorization>
< ...