How to Modifying app.config During Installation

//Usage
WriteSettings("KeyInAppConfig", "NewValues");
//This will find the KeyInAppConfig setting in app.config and set the value of it to NewValues

private static XmlDocument loadConfigDocument()
{
	XmlDocument doc = null;
	try
	{
		doc = new XmlDocument();
		doc.Load(getConfigFilePath());
		return doc;
	}
	catch (System.IO.FileNotFoundException e)
	{
		throw new Exception("No config filed found.", e);
	}
}

private static string getConfigFilePath()
{
	return Assembly.GetExecutingAssembly().Location + ".config";
}


public static void WriteSetting(string key, string value)
{
	XmlDocument doc = loadConfigDocument();
	XmlNode node = doc.SelectSingleNode("//appSettings");

	if (node == null)
		throw new InvalidOperationException("appSettings section not found in config file.");

	try
	{
		XmlElement elem = (XmlElement)node.SelectSingleNode(string.Format("//add[@key='{0}']", key));

		if (elem != null)
		{
			elem.SetAttribute("value", value);
		}
		else
		{
			elem = doc.CreateElement("add");
			elem.SetAttribute("key", key);
			elem.SetAttribute("value", value);
			node.AppendChild(elem);
		}
		doc.Save(getConfigFilePath());
	}
	catch
	{
		throw;
	}
}