modifying an xml file

To read the xml file :

XmlDocument xd = new XmlDocument();

xd.Load(filename);

To save the file:

xd.Save(filename);

but you'll have issues saying File is being used by other process - "read more about this in the link below"; 

So, create a filestream to be used. 

XmlDocument xd = new XmlDocument();

FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);

xd.Load(fs);

fs.Seek(0, SeekOrigin.Begin);

fs.SetLength(0);

xd.Save(fs);

Fore more detailed explanation check this link: 

http://geekswithblogs.net/chrisfalter/archive/2006/03/27/73515.aspx