update xml file

Hi,
Using the xmlWriterSetting and xmlreaderSettings, I am saving and reading xml files...
So if a new xml data needs to be added to the xml file, I use the xmlWriterSetting to save into the xml file.

Questions:
1)
How is it possible to enhance what I have so that I can UPDATE an xml file?
2)
Do I have to use xmlDocument?
3)
Can I update using the xmlWriterSetting?

Thanks