Many users are frustrated that they are stuck downloading the Windows 8.1 update via the Windows Store and the option to download an ISO is unavailable. Microsoft revealed that they would not be providing an ISO for download, but that doesn't mean we can't try some tricks and obtain an ISO!
A user on Reddit apparently figured out a little trick that lets you download the Windows 8.1 ISO using a legitimate retail Windows 8 product key. We will list the steps needed in order to reproduce this yourself. First, head over here and download both the Windows 8 and Windows 8.1 setup installers.
Once you download both installers, launch the Windows 8 installer and enter your product key when prompted to do so. Once the download begins, immediately pause your download at 1%. Do not quit the installer at the estimating time screen. Once you pause the download at 1%, quit the installer.
Now, run the Windows 8.1 installer and you will notice that it will tell you "download did not complete successfully" and that there was an "element not found" error. This is normal. Close the installer. Now, re-launch the same Windows 8.1 installer. You will now be downloading the Windows 8.1 ISO. After the download is complete, you can save it as an ISO or create the USB media immediately.
It appears that this method is indeed working. You can install Windows 8.1 using the ISO or even burn it to a DVD or make a bootable USB thumb drive with the media. Try it out and let us know below if you were successful in doing so.