Do you know how to read an XML file into "objects" (create a XmlDocument ... or DOM ... from the XML) ? That's what you will need to do to change a specific property or state; and then, you'll need to save the changed DOM back to the file to update the file.
I suggest you start by doing some basic reasearch on Objects-to-XML and XML-to-Objects; there are several good articles here on CodeProject, including: [^
This CP article has just been published this month: [^
]. Unfortunatenly this article does not cover the use of DataContract, which is the newest (included in .NET 3.0 and later), and, imho, best way to move objects to and from XML: [^