How do I change a value in web.config from code?

 I used formauthentication for admin and user side login.i can login to admin as i given <location path="admin">
    <system.web>
      <authorization>
        <allow roles="admin"/>
        <deny users="?"/>
      </authorization>
    </system.web>
  </location>
 
but for userside ther may more users exist then how can i do the login???
any body pls help

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