using System.Configuration;
      

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

config.AppSettings.Settings[
      
      
        "
      
      
        defaultPath
      
      
        "
      
      ].Value =
      
         value;

config.Save();

ConfigurationManager.RefreshSection(
      
      
        "
      
      
        appSettings
      
      
        "
      
      );