This is an epic trick! Apparently there is a workaround to download the Windows 8.1 ISO from the Microsoft servers using your Windows 8 product key — Yes, even the upgrade key works with this method.
Why is this is a good news? Because Microsoft has recently announced that the only way to update to Windows 8.1 is via the Windows Store and it won’t be making available an ISO file to the public, only for TechNet and MSDN subscribers. And let’s say you had a small business with 20 PCs, you’ll literally have to manually update each computer using the Store, downloading the installation files of about 3.5GB 20 times — now you’ll only have to do it once.
Follow these steps to the image file:
2 Scroll down and click Install Windows 8 (This will trigger the download for the setup assistant).
3 Launch the installer and enter your Windows 8, Windows 8 Pro or Windows 8 Upgrade product key, confirm that you will be downloading the bits for the version you paid for.
4 As soon as starts downloading, close the installer assistant from the (X) in the top-right corner.
5 Go back to the Microsoft Windows upgrade page, click Install Windows 8.1, and download the Windows 8.1 Setup assistant.
6 Now like if it was magic, you’ll see that Windows 8.1 starts downloading… (For those non-believers I tested by letting the first download go to 5%, then I switched to the 8.1 download and it started again from 0%.)
7 When the download completes, you can choose to install the OS right-then, or you can choose to the option: Install by creating media. (This option will allow you to create a bootable media.)
8 Choose to create a USB flash drive or ISO file, and click Next to finish.
Even better this will also solve the “Insert Media: Some files are missing…” issue while using Refresh your PC or Remove everything in Windows 8.1.
Kudos to Nazmus Khandaker for publishing this helpful tip that he found in the Microsoft forums.
Update: Here are a few tips you can try, if Windows 8.1 isn’t taking your Windows 8 retail key.
Update: I created a video to make a bit easier to understand how the download works.
Update: For those who downloaded Windows 8.1 official ISO and can’t install it with the Windows 8 retail product key. This guide will tell you all the steps to bypass the product key and delay the Windows activation for up to 30 days.