Reading from App.config file:
using System.Configuration;

To read from app.config file you may use the following code:
private string ReadFromAppConfig(string key)
{
 return ConfigurationManager.AppSettings[key];
}

To write to app.config file you may use the following code:
private void WriteToAppConfig(string key, string value)
{
 Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
 config.AppSettings.Settings[key].Value  = value;
 config.Save(ConfigurationSaveMode.Modified);
 ConfigurationManager.RefreshSection("appSettings");
}


Sample App.Config file:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="TimerInterval" value ="9000"/>
  </appSettings>
</configuration>