Update a ConnectionString Stored in App.Config File

        using System.Configuration;
        public void updateConnectionString()
        {
           //create a new connection string settings
            ConnectionStringSettings cSetting = new ConnectionStringSettings();

            //connection string
            cSetting.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\Database1.mdf;Integrated Security=True;User Instance=True";
            //connection settings name in conf. file
            cSetting.Name = "WindowsFormsApplication1.Properties.Settings.conStr";
            //Provider
            cSetting.ProviderName = "MySql.Data.MySqlClient";

            //get the application conf file
            Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
           
            //read the connection string section
            ConnectionStringsSection connectionSettings = (ConnectionStringsSection)config.GetSection("connectionStrings");

            //remove the current conf section and add new
            connectionSettings.ConnectionStrings.Remove(cSetting);
            connectionSettings.ConnectionStrings.Add(cSetting);

            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection(config.AppSettings.SectionInformation.Name);

            //to read and view
            //string newString = ConfigurationManager.ConnectionStrings["WindowsFormsApplication1.Properties.Settings.conStr"].ConnectionString;
            //System.Diagnostics.Debug.Print(newString);
        }