As you're likely already aware, Windows 8.1 is officially complete and released to the public. Though users looking to upgrade from earlier versions of Windows will obviously have to pay for it, Windows 8.1 is free of charge to existing users of its predecessor Windows 8. Such users should be able to simply download the update via the Windows Store and install it without any fuss at all, but that isn't working out for a lot of users.
If you're currently running Windows 8 and you can't seem to find 8.1 in the Store, don't panic. Don't even run for the ISO downloads and install that way. All you need is a quick update for the Store and you'll be ready to go. For most users, the update should install automatically, but if you aren't configured for automatic updating, you'll need to get it yourself.
To trigger an update you'll need to click "Change PC Settings" from the Settings charm. Select "Windows Update" from the left pane of app that opens up and click "Check for updates now." When you get results back stating that there are indeed updates available, click "We'll install ## updates automatically."
You'll now see a list of updates available for installation. You can simply click "Install" to download and install them all, but if you're behind, that could take a while. For a faster fix, click "Choose which important updates you want to install to be taken to the Control Panel. Once there, click "## important updates are available. Deselect all but KB2871389 and click "Install." This update is less than 10 MB and should only take a minute or so. Once done, restart your computer and try the Store again. You should find a link to Windows 8.1 waiting for you.
If you know of anyone experiencing this issue, make sure to share this with them.
Image above courtesy of Microsoft.