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.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: Resetting the value of an attribute which is already set.