XSLT is a powerful tool for transforming one XML document into another. A typical example is to rewrite an XML returned from a database to a human-browsable (X)HTML representation.
Taverna comes with an XSLT processor, located under Available Services > Local Services > xml >Transform XML or Transform XML with parameters
The output can be retrieved both as a real value for your workflow and/or stored in a file.
You should try manually with a command line XSLT tool (like
xsltproc) or similar before using it from Taverna, as XSLT might be a difficult language. One recommendation is to try to find an XSLT stylesheet that does almost what you want already, and then modify it to your needs.