Change order of XML using XDocument

<root>
  <one>1</one>
  <two>2</two>
</root>

public static class XElementExtensions
{
    public static void OrderElements(this XElement parent, params string[] orderedLocalNames)
    {
        List<string> order = new List<string>(orderedLocalNames);
        var orderedNodes = parent.Elements().OrderBy(e => order.IndexOf(e.Name.LocalName) >= 0? order.IndexOf(e.Name.LocalName): Int32.MaxValue);
        parent.ReplaceNodes(orderedNodes);
    }
}
// using the extension method before persisting xml
this.Root.Element("parentNode").OrderElements("one", "two", "three", "four");

static void Main(string[] args)
{
    XDocument doc = new XDocument(
        new XElement("root",
            new XElement("one", 1),
            new XElement("two", 2)
            ));

    var results = from XElement el in doc.Element("root").Descendants()
                  orderby el.Value descending
                  select el;

    foreach (var item in results)
        Console.WriteLine(item);

    doc.Root.ReplaceAll( results.ToArray());

    Console.WriteLine(doc);

    Console.ReadKey();
}

XElement node = ...get the element...

//Move up
if (node.PreviousNode != null) {
    node.PreviousNode.AddBeforeSelf(node);
    node.Remove();
}

//Move down
if (node.NextNode != null) {
    node.NextNode.AddAfterSelf(node);
    node.Remove();
}