There is an upgrade package available but it upgrades the entire installation. They strongly recommend that you backup everything before using it.
http://www.apachefriends.org/en/xampp-windows.htmlUpgrading only PHP from a standard PHP package may not work. XAMPP typically is installed in it's own directories that are different from what a plain PHP installation usually uses. The ini files are also in a different location. And there are registry entries for the installations.