Upgrading your Windows 8 system to Windows 8.1 is simple enough through the Microsoft Store, but if you need to reinstall it or run the update on more than one PC, it can be inconvenient and time-consuming since Microsoft doesn't give you the option to save the update files.
There's a trick, however, that allows you to download the official Windows 8.1 ISO from Microsoft's servers with your Windows 8 product key. With the Windows 8.1 ISO, you can create DVD or USB media to update a Windows 8 PC to Windows 8.1. Unfortunately, as Paul Thurrott over at WinSupersite mentions, you can't use the ISO to perform a clean install of Windows 8.1 with a Windows 8 product key. The ISO is still worth having, though, just to be able to bypass the Microsoft Store when upgrading to Windows 8.1 from Windows 8. Here's how to download the official Windows 8.1 ISO:
Step 1: Go to Microsoft's page for upgrading to Windows 8 with a product key, then click on the light blue "Install Windows 8" button.
Step 2: Launch the setup file (Windows8-Setup.exe) and enter your Windows 8 product key when prompted. Continue the setup process until it begins downloading Windows 8.
Step 3: Close the Windows 8 setup window and confirm that you want to quit the setup.
Step 4: Back at the Microsoft upgrading to Windows 8 with a product key page, click on the light blue "Install Windows 8.1" button this time.
Step 5: Launch the setup file (WindowsSetupBox.exe), which will begin downloading the Windows 8.1 ISO.
Step 6: Once the ISO finishes downloading, the installer will take several minutes to check the download and uncompress the install files. When you're prompted to choose an installation option, pick "Install by creating media."
Step 7: Decide whether you want the installer to create USB flash drive media, or save the ISO file.
If you choose ISO, the ISO file (Windows.iso) will be saved to your PC and you'll be given the option to burn it to DVD. Once completed, you can actually go back and create the USB flash drive media as well. Just click on the "Install Windows" link on your Windows desktop. You'll be prompted to choose your installation option again. Pick "Install by creating media" again, insert a 4GB or larger USB flash drive, then choose the USB flash drive option. If you have any problems creating the USB flash media, you can still use the Windows 7 USB/DVD download tool to create USB media with the Windows 8.1 ISO.