Refer following code :-
string szFilePath = @"D:\ABC.xml";
XmlDocument objDOM = new XmlDocument();
objDOM.Load(szFilePath);
int iIndex = 0;
foreach (XmlNode objXNode in objDOM.GetElementsByTagName("Model")[0].ChildNodes)
{
if (objXNode.Name.Trim() == "Component")
{
objXNode.Attributes["id"].Value = (iIndex).ToString();
iIndex++;
}
}
objDOM.Save(szFilePath);