Here is some sample code:

// Reading a key
        String Version = ConfigurationManager.AppSettings["VersionOfWebSite"];


        // Writing a key
        ExeConfigurationFileMap FileMap = new ExeConfigurationFileMap();
        FileMap.ExeConfigFilename = Server.MapPath(@"~\Web.config");


        Configuration Config = ConfigurationManager.OpenMappedExeConfiguration(FileMap, ConfigurationUserLevel.None);


        Config.AppSettings.Settings.Add("Test", "TestValue");
        Config.Save(ConfigurationSaveMode.Modified);