Modify Web.Config AppSetting Value

Main()

{
Configuration config = OpenConfigFile(Server.MapPath("~/web.config"));

            config.AppSettings.Settings["XmlFile"].Value = "UrlRoute"+(Convert.ToInt32(fileno)+1).ToString()+".xml";

            config.Save(ConfigurationSaveMode.Modified);

            ConfigurationManager.RefreshSection("appSettings");
}

public static Configuration OpenConfigFile(string configPath)

    {

        var configFile = new FileInfo(configPath);

        var vdm = new VirtualDirectoryMapping(configFile.DirectoryName, true, configFile.Name);

        var wcfm = new WebConfigurationFileMap();

        wcfm.VirtualDirectories.Add("/", vdm);

        return WebConfigurationManager.OpenMappedWebConfiguration(wcfm, "/");

    }