Using LINQ to xml if you are using framework 3.5
using System.Xml.Linq;
XDocument xmlFile = XDocument.Load("books.xml");
var query = from c in xmlFile.Elements("catalog").Elements("book")
select c;
foreach (XElement book in query)
{
book.Attribute("attr1").Value = "MyNewValue";
}
xmlFile.Save("books.xml");