Following is the simple code to change the value of the appSettings key,
private void UpdateConfig(string strKey, string strValue) { Configuration objConfig = WebConfigurationManager.OpenWebConfiguration(”~”); AppSettingsSection objAppsettings = (AppSettingsSection)objConfig.GetSection(”appSettings”); if (objAppsettings != null) { objAppsettings.Settings[strKey].Value = strValue; objConfig.Save(); } }