In XSLT 2.0, use regular expressions.
In 1.0, You just need to pick out the components of the date using substring-before and substring-after, e.g. the second component is substring-before(substring-after($in, '/'), '/'). Then you can use format-number to add a leading zero, and concat() to put the three parts together in the right order.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference