Modify Web.config

private voidChangeAppSettings(string key,stringNewValue) {    

  Configuration cfg;    

  cfg = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");     

      KeyValueConfigurationElement setting= (KeyValueConfigurationElement)cfg.AppSettings.Settings(key);      

     if((setting !=null)){       

   setting.Value= NewValue;         

 cfg.Save();    

  } 

 }