If you need to reinstall Windows 8.1 or you want to install it on a partition and you don't have installation media, you can create it using the Windows 8.1 Setup program. To create media you need to use a PC running Windows 8.1, Windows 8, or Windows 7, and it must have the same architecture (32 or 64-bit) as the PC you want to install Windows 8.1 on. You'll also need your Windows 8.1 product key.
You can usually find your product key in your online order info, in the confirmation email for your purchase, or on the DVD packaging. The product key will look something like this:
XXXXX- XXXXX- XXXXX- XXXXX- XXXXX.
If you don't have a Windows 8.1 product key, you can't create installation media. You can't use a Windows 8 product key for this.
Make sure the USB flash drive or DVD you're using has nothing else stored on it, and has enough free space to download the Windows 8.1 Setup program: about 1.8 GB for the 32-bit version and about 2.3 GB for the 64-bit version.
Once you have your product key and are on a PC running Windows 8.1, Windows 8, or Windows 7, follow these steps:
Run the Windows 8.1 Setup program.
When prompted to run or save the file, choose Run
When prompted to enter a product key, enter the 25-digit product key you received when you bought Windows 8.1.
When prompted to install Windows 8.1, select Install by creating media, and then click Next.
Select the type of media you want to use: either a USB flash drive or a DVD.
Click USB flash drive.
Plug the USB flash drive into a USB port on your PC.
When prompted, select the USB drive you want to use and then click Next.
If you have files on the USB flash drive, they'll be deleted.
Click ISO file and then select the location to save the ISO file.
Insert a blank, recordable DVD into your DVD drive.
Burn the ISO file to the DVD using Windows Disc Image Burner, or another DVD burning program.
On Windows XP and Windows Vista, you can't burn an ISO file to a DVD or USB drive. For more info, go to this Microsoft Support article about creating installation media on Windows XP.