To install Windows 8.1 on another partition:
You must have a version of Windows already running in the partition.
You must create a DVD or USB flash drive that you can boot from.
If you're using an ISO file to install Windows, you must burn the ISO file to a DVD. You can't use a USB flash drive for this.
Write down your Windows 8.1 product key. You won't be able to install Windows 8.1 without it.
After you create the DVD or USB flash drive, you’ll boot from it, and then choose a partition to upgrade to Windows 8.1.
Some older PCs might not be able to boot from a USB flash drive. To learn whether your PC can boot from a USB flash drive, check the info that came with your PC or the manufacturer's website.
If you're trying to install the free update from Windows 8 to Windows 8.1 and you don't have a Windows 8.1 product key, you can't boot from media to install this update.
Turn on your PC and insert the DVD or USB flash drive when you're ready to install Windows.
Restart your PC.
If you see a message asking you to “Press any key to boot from DVD,” do so. If you don't see this message, or if your current version of Windows starts over and over again, you might need to change the boot order in your PC's BIOS settings.
To change the boot order in your PC's BIOS settings, you generally need to restart your PC and then immediately press a combination of keys (such as F2, F12, Delete, Esc). Because this key combination varies from one PC to another, you might need to visit your PC manufacturer's website to learn the key combination you'll need for your PC.
If you restart and press the required key combination, but the current version of Windows still starts, wait until Windows asks you to sign in, and then restart your PC to try again.
Changing the boot order is intended for advanced users only, because it's possible to unintentionally change something that could prevent your PC from starting correctly.
On the Install Windows page, click Install now, and follow the steps.