Excellent.
Browse to the installation media and run setup.
When asked, choose Download and install updates (recommended), and then click Next.
Enter your product key when it is requested, then click Next.
Accept the license terms.
The very important step here is to choose the option titled 'Keep Windows settings, personal files and apps'. Be absolutely certain that this option is selected and then click Next.
Click install when prompted. Windows 8.1 setup should begin.