Microsoft doesn’t have a direct way of downloading a copy of Windows 8.1 if you already bought a copy of Windows 8, or if you have your own legitimate key for it. Fortunately, there’s a workaround that works well.
One reason you may want to do this is when you want to do a clean installation of Windows, but want to go straight to 8.1, rather than using your existing copy of Windows 8 and later installing all of the updates.
I recently did just that. My existing copy of Windows 8 was problematic, and wouldn’t let me install the Windows 8.1 update from the Store app. Aside from that, everything else about the OS worked just fine. I finally wanted to go to 8.1, but the only way to do that was to download a raw copy of 8.1. Since I only had my Windows 8 key, I had to follow this workaround to get the 8.1 ISO directly from Microsoft. Afterwards, I was able to upgrade my copy of Windows 8 using said ISO file without any issues. For more information on that, view this tutorial.
(Update on 5/19/2014: It looks as though Microsoft changed it so that it doesn’t pick up where the Windows 8 downloader left off, instead prompting you for a Windows 8.1-specific key. To work around this, simply go to the tutorial linked above (or just click here) to get a “generic” Windows 8.1 key that will allow you to download the ISO like usual. Because of this change, you can ignore the first step below.)
(Update 7/30/2014 — The Windows 8.1 downloader doesn’t work as initially intended. Why Microsoft changed it is questionable. You can download the original one here, ignoring the “that page” link below in step 2.)
First, navigate to this Microsoft page, titled “Upgrade Windows with only a product key.” From there, scroll down a bit, and click on “Install Windows 8.” This will download a small program which will download the Windows 8 ISO using your key. We’re picking this one instead of the Windows 8.1 option because your Windows 8 key won’t work, since it’s for just that–Windows 8. After it downloaded, run it, and enter your Windows 8 key. Once it begins the download, close out of the program.
Next, navigate back to that page (again, see the 7/30/2014 update above), but this time, click on “Install Windows 8.1.” After it downloads, open it up, and it will “resume” the download, although it will be downloading Windows 8.1, rather than what it was originally downloading, which was Windows 8.
When it’s done downloading, you’ll be prompted with options to save the download as an ISO file, or have all the files copied over to a USB drive. If you go with the former, you can do the same thing with Microsoft’s free “Windows 7 USB/DVD download tool,” including to DVDs.
And that’s it. Why Microsoft doesn’t have a direct way to get Windows 8.1 using your Windows 8 key, I don’t know. Given that Windows 8.1 is a free update, it’s even more of a head-scratcher.
This method has been available for quite some time, and most likely won’t be patched up anytime soon.