Update App.Config Programmatically

ExeConfigurationFileMap fileMap = new ExeConfigurationFileMap();

fileMap.ExeConfigFilename = @"YourAppFile.exe.config";

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

AppSettingsSection app = config.AppSettings;

app.Settings.Remove("key ");

app.Settings.Add("key", "value");

config.Save(ConfigurationSaveMode.Modified);