This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Do you mean, after you have used an XSL transformation including that line to output an XML document, you want to change that attribute? Well, another XSL transformation could do it. Or you could load it into a DOM and change the attribute there. Or... plenty of other ways.
Or do you mean something else? There's nothing special about using <xsl:attribute> to add an attribute to an element.