In order to install Windows 8.1 64bit operating system, insert the Windows DVD or USB flash drive and reboot your computer. After that, press any key to open the boot menu, enter your language, select the other preferences and click Next. Press Install Windows, type the product key (XXXXX-XXXXX-XXXXX-XXXXX-XXXXX), check the I accept the license terms box from the Please read the license terms page and click again Next. After that, select Custom from the Which type of installation do you want? window, tap or click the partition that you want to use and follow the on-screen instructions. For further information, check Microsoft Support page. Also, if you already have Windows 8, and you want to update to Windows 8.1, follow the instructions provided by the developers.