string te = ConfigurationManager.AppSettings["TestEnvironment"];

Configuration cfg = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

string configPath = @"TE\environment_new.xml";
Configuration cfg = ConfigurationManager.OpenExeConfiguration(configPath);

<configuration>
...
   <TestEnvironment configSource="TE\environment_new.xml" />
...
<configuration>

string environmentPostfix, newConfigSourceText;
            

 // Get environment suffix from config
            XmlDocument configXml = new XmlDocument();
            configXml.Load(ConfigFile);
            XmlNode configCurrentEnvironmentNode = configXml.SelectSingleNode("/Config/CurrentEnvironment");
            environmentPostfix = configCurrentEnvironmentNode.InnerText;

            // Get current environment from app.config
            Configuration cfg = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            string configSourceText = cfg.Sections["TestEnvironment"].SectionInformation.ConfigSource;

            // Replace current ConfiGSource text according to loaded changes
            int firstpos = configSourceText.IndexOf("_");
            int lastpos = configSourceText.IndexOf(".");
            newConfigSourceText = configSourceText.Substring(0, firstpos + 1); //index is zero-based, so increment is required
            newConfigSourceText += environmentPostfix;
            newConfigSourceText += configSourceText.Substring(lastpos, configSourceText.Length - lastpos);
            cfg.Sections["TestEnvironment"].SectionInformation.ConfigSource = newConfigSourceText;

            // Save configuration file
            cfg.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("TestEnvironment");