Updating connection string in app.config permanenlty from c#

static void Main(string[] args)
{
    XmlDocument xmlDoc = new XmlDocument();
    xmlDoc.Load(AppDomain.CurrentDomain.BaseDirectory +
                                    "..\\..\\App.config");
    XmlNode appSettingsNode = xmlDoc.SelectSingleNode("configuration/appSettings");
    // Attempt to locate the requested setting.
    foreach (XmlNode childNode in appSettingsNode)
    {
        if (childNode.Attributes["key"].Value == "test")
            childNode.Attributes["value"].Value = "test1";
    }
    xmlDoc.Save(AppDomain.CurrentDomain.BaseDirectory +
                                    "..\\..\\App.config");
    xmlDoc.Save(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);

    Console.Read();
}
only make sure application will require read/write permissions on the App.config in order to save changes.

hope this help.