Download the Windows 8.1 ISO using a Windows 8 key and some simple trickery
It’s great that Microsoft has made Windows 8.1 entirely free to owners of Windows 8, but unfortunately you’re limited to installing the update through the Windows Store -- unless you’re an MSDN subscriber there’s no ISO you can grab to install the operating system how and when you like.
But as enterprising Reddit user redditorfor0seconds has discovered, there is a way of getting your hands on the ISO that only requires you to have a Windows 8 retail or MSDN key (but not an OEM key, apparently). It’s incredibly straightforward too, just follow these simple instructions.