[Test]

        public void XPathTest()

        {

            var path1 = "/configuration/appSettings/add[@key='OutputRuleWebPartExecutionTimeElapse']";

            var path2 = "/configuration/appSettings/add[@key='OutputRuleWebPartExecutionTimeElapse']/@value";

            var desktopPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);

            var fileName = "test.xml";

            var filePath = Path.Combine(desktopPath, fileName);

            var doc = new XmlDocument();

            doc.Load(filePath);

            var node1 = doc.SelectSingleNode(path1);

            var node2= doc.SelectSingleNode(path2);

            Console.WriteLine(node1.OuterXml);

            Console.WriteLine(node2.OuterXml);

            Console.WriteLine(node2.Value);

        }

<?xml version="1.0" encoding="utf-8"?>

<configuration>

  <appSettings>

    <add key="OutputRuleWebPartExecutionTimeElapse" value="False" />

  </appSettings>

</configuration>

XmlDocument document = new XmlDocument();

document.Load(fileName);

XmlNode node = document.SelectSingleNode("Elements/Element1");

node.ParentNode.RemoveChild(node);

document.Save(fileName);