sorting xml file using linq

Something like the following. You should properly use int.TryParse instead of int.Parse and some other checks (for specific elements etc.) if your are not 100% sure of the input. Or check it against a schema.

(I have added a person in the end with id=1. I guess your forgot to add it.)

var doc = XDocument.Parse(@"<Persons>
<Person>
<id>2</id>
<Name>xxx</Name>
</Person>
<Person>
<id>3</id>
<Name>yyy</Name>
</Person>
<Person>
<id>1</id>
<Name>some name</Name>
</Person>
</Persons>");

//By building a new XDocument
var newDoc = new XDocument(new XElement("Persons",
                from p in doc.Element("Persons").Elements("Person")
                orderby int.Parse(p.Element("id").Value)
                select p));
//or by changing the original XDocument (does not change it where you got it from - a file etc.):

doc.Root.ReplaceAll(from p in doc.Root.Elements("Person")
                    orderby int.Parse(p.Element("id").Value)
                    select p);

And to load/save your can use:

var doc = XDocument.Load("filename"); //Or use a stream etc.
newDoc.Save("filename"); //Or save to a stream etc.

You can use:

doc.Root

instead of:

doc.Element("Persons")

But again, check the elements if your not 100% sure of the input.