How to Update an element of xml value without loading the whole xml document?

Here is how it will work :

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load(@"C:\Well.xml");
XmlNode node = xmlDoc.SelectSingleNode("/gpx/waypoints/waypoint[@a='2']/name");
node.InnerText = "Something";
xmlDoc.Save(@"C:\Well.xml");

Hope it helped! :)

Update 1:
From the subject of your question, it seems that you don't want to load the whole XML.
Well there is a class XmlTextWriter which is a stream-based loading of XML and it won't load the whole XML. It will read node by node, in contrast to XmlDocument which is a in-memory based loading of XML, it loads the whole XML into memory.

Now coming back to the point, after searching for more info on the internet, it was just like I thought, XmlTextWriter cannot modify the Xml nodes. So your solution will be to use the XmlDocument itself for Modifying the nodes.

I did however tried in this way :

XmlTextReader reader = new XmlTextReader(path);
while (reader.Read())
{
    if (reader.NodeType == XmlNodeType.Element && reader.Name == "waypoint")
    {
        if (reader.GetAttribute("a") == "2")
        {
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.LoadXml(reader.ReadOuterXml());
            XmlNode node = xmlDoc.SelectSingleNode(".");
            //String str = node.InnerText;
            node.InnerText = "Something";
            //The above statement gave this error : Object reference not set to an instance of an object.
        }
    }
}

:(

So it didn't work for me.
I will still try to figure out a solution for this.

Update 2:

Found some articles in CodeProject. According your requirements, check these :

-First you can split your large xml file into smaller ones. Check this article : http://www.codeproject.com/KB/XML/SplitLargeXMLintoSmallFil.aspx#xx2994884xx[^]

- After finding the node, load the smaller XML into an XmlDocument and then edit like I did in my solution. Then merge the xml files like mentioned in this article : http://www.codeproject.com/KB/XML/XStreamingElement_Merge.aspx[^]

Try it out. Good luck!