Xsl consists of three parts: Xslt, Xpath, Xsl-fo.
None of these languages can be used to "update" an xml document.
To my knowledge, you will require some server-side programming to achieve your task.
There should be many examples of how to update xml documents with server-side programming on the internet.
You can also ask in the .net forum, as I am sure Microsoft will have a very simple way to manipulate xml documents.