An update from one of our product marketing experts:
Windows 8.1 was released today, and we are happy to share that we released an update for Parallels Desktop 9 for Mac — so, now you can update your Windows 8 virtual machines to Windows 8.1. The best approach is to:
(1) update Parallels Desktop 9, then,
(2) boot your Windows 8 virtual machine — which will then automatically update the Parallels Tools inside the virtual machine — and then,
(3) upgrade Windows 8 to Windows 8.1.