Syntax:
UpdateXML(xml_target, xpath_expr, new_xml)

This function replaces a single portion of a given fragment of XML
markup xml_target with a new XML fragment new_xml, and then returns the
changed XML. The portion of xml_target that is replaced matches an
XPath expression xpath_expr supplied by the user. If no expression
matching xpath_expr is found, or if multiple matches are found, the
function returns the original xml_target XML fragment. All three
arguments should be strings.

URL: http://dev.mysql.com/doc/refman/5.1/en/xml-functions.html

