How to modify my App.exe.config keys at runtime

On some occasions we need to store values ​​in app.config file confirguracion from our own application at runtime. 

Without giving a lot of thought I will show one of the methods that I used to do, and is as follows:

System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

    config.AppSettings.Settings["Test"].Value = "valueTest";
    config.Save(ConfigurationSaveMode.Modified);