Update app.config system.net setting at runtime

I need to update a setting in the system.net SectionGroup of a .Net exe app.config file at runtime. I don't have write access to the original config file at runtime (I am developing a .Net dll add-in which is hosted in an exe provided by the app which I have no control over) so I was hoping to save a copy of the file and replace the config in the exe with the modified version at runtime. I've tried the following but it's not working. Any suggestions?

    Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    NetSectionGroup netSectionGroup = config.GetSectionGroup("system.net") as NetSectionGroup;
    netSectionGroup.Settings.HttpWebRequest.UseUnsafeHeaderParsing = true;                      
    config.SaveAs(@"C:\ProgramData\test.config", ConfigurationSaveMode.Full);

    AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", @"C:\ProgramData\test.config");

I did not understand from your question if you don't have access to the app.config file because of your own design implementation or you just weren't able to save the config file, so here is a piece of code that allows you to modify and save appSettings section in the config file at runtime:

Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
KeyValueConfigurationCollection settings = config.AppSettings.Settings;

// update SaveBeforeExit
settings[-keyname-].Value = "newkeyvalue";
...
//save the file
config.Save(ConfigurationSaveMode.Modified);
//relaod the section you modified
ConfigurationManager.RefreshSection(config.AppSettings.SectionInformation.Name);

P.S the code will not save the app.config file you see in the solution editor, it will pdate the "program_name.exe.config" file in the operation forlder.

using System.Configuration;

    public void save_new_connection()
    {

      string ConStrng = ConfigurationManager.ConnectionStrings.ToString();
      ConnectionStringSettings conSetting = new ConnectionStringSettings();

      conSetting.ConnectionString="server=localho;UserId=root;password=mypass;database=night_anglecourier"; 
      conSetting.Name = "courier.Properties.Settings.night_anglecourierConnectionString";
      conSetting.ProviderName = "MySql.Data.MySqlClient";

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

        ConnectionStringsSection conSettings = (ConnectionStringsSection)config.GetSection("connectionStrings");
        conSettings.ConnectionStrings.Remove(conSetting);
        conSettings.ConnectionStrings.Add(conSetting);

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


    }

with this code i have changed the connection string in the application setting of the config file ... hope this may help u.

string ConStrng = ConfigurationSettings.AppSettings["ConnectionString"];
            string sss = "Data Source=";
            string xxx = ";Initial Catalog=AlfalahScholarship;Integrated Security=True";
            //ConfigurationSettings.AppSetting;
            System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            //Get the appSettings section.
            AppSettingsSection appSettings = (AppSettingsSection)config.GetSection("appSettings");
            appSettings.Settings.Remove("ConnectionString");
            appSettings.Settings.Add("ConnectionString", sss + txtServerName.Text + xxx);

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