Some Windows 8 and RT users may need an update before 8.1 can be found via the store. Luckily Microsoft have a blog released so users know what to look for:

Why can't I find the update in the Store?

You might need the latest updates from Windows Update Although the Windows 8.1 update is downloaded and installed from the Store, your PC needs another update, KB 2871389, before Windows 8.1 becomes available to you in the Store.

If you’re using Windows Update to get automatic updates, KB 2871389 will be downloaded and installed automatically.

If you’ve turned off automatic updates, or if your PC is so new that automatic updates haven't been downloaded yet, or if you’d rather not wait for Windows Update to do this automatically, you can go to Windows Update and install KB 2871839 manually.

To install the update you need
1.Open Windows Update by swiping in from the right edge of the screen (or, if you're using a mouse, pointing to the lower-right corner of the screen and moving the mouse pointer up), tapping or clicking Settings, tapping or clicking Change PC settings, and then tapping or clicking Update and recovery.

2.Tap or click Check now, and then wait while Windows looks for the latest updates for your PC.

3.If updates are found, tap or click View details.

4.In the list of available updates, select KB 2871389, and any other updates you want to install, and then tap or click Install.

5.Read and accept the license terms, and then tap or click Finish, if the updates you’ve selected require it.

You might need to restart your PC to finish installing some updates. Save and close all your files and apps before you restart so you don't lose anything.

After KB 2871389 is installed and you’ve restarted your PC, go to the Start screen, tap or click Store, and look for theWindows 8.1 update. For more complete info on installing Windows 8.1, see Update to Windows 8.1 from Windows 8.

You might be running an edition of Windows 8 that doesn’t support the free update from the Store Some editions of Windows 8 don’t support the free update to Windows 8.1. These include:

Windows 8 Enterprise

Editions of Windows 8 Pro that are installed by enterprises using KMS activation

Editions of Windows 8 that are installed using an MSDN ISO, and activated using multiple activation keys

If you’re running one of these editions, you can't install the free update to Windows 8.1 or Windows RT 8.1 from the Store.

If you installed Windows 8 using an MSDN ISO, you might be able to install Windows 8.1 using a similar ISO from MSDN.

Otherwise, you might be able to perform a clean install. For more info, see How to perform a clean installation of Windows.

For more info about updating to Windows 8.1, see Update to Windows 8.1: FAQ.

If you're updating from Windows 8.1 Preview or Windows RT 8.1 Preview, see Update from Windows 8.1 Preview to Windows 8.1.

Why can't I find the Win 8 update in the Store