If you are having problems finding the FREE Windows 8.1 Upgrade in the store, this is the best fix I have found.
Note: This is only available if the version of windows is OEM, not an after market install.
The Windows 8.1 Upgrade hinges primarily around a certain Windows Update: (KB2871389). If this update is not installed, the update will not show up. If this update is installed and the store still shows nothing, uninstall this particular update, reboot, and run windows updates again. Windows will find and reinstall this update and the Windows 8.1 Upgrade should now be found in the store.
You may also go to Run and type wsreset.exe. This will clear the cache for the store and the update may show up.
Hope this helps!