Modify your configuration file

Add "System.Configuration" reference to your application


 ExeConfigurationFileMap configFileMap = new ExeConfigurationFileMap();      
            configFileMap.ExeConfigFilename = @"{path}";

            Configuration config = ConfigurationManager.OpenMappedExeConfiguration(configFileMap, ConfigurationUserLevel.None);


            Console.WriteLine(config.AppSettings.Settings["YourKey"].Value);
            config.AppSettings.Settings["YourKey"].Value = DateTime.Now.Date.ToString("yyMMdd");
            config.Save();