ConfigurationManager

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="Number0" value="9759436"/>
    <add key="Number1" value="9759437"/>
    <add key="Number2" value="9759438"/>
    <add key="Number3" value="9759439"/>
  </appSettings>
    <startup>
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

private void Button_Click_1(object sender, RoutedEventArgs e)
 {
        int i = int.Parse(ConfigurationManager.AppSettings["Number2"]);
        this.txtMsg.Text = i.ToString();
 }

Configuration cfa = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
cfa.AppSettings.Settings["Number2"].Value = "65434";
cfa.Save();