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.

If the upgrade shows up in the Windows Store:

Once you have logged into windows for the first time (with the default user), the Windows 8.1 upgrade will show up in the Windows Store, but will not run due to an error “…must be an administrator…”.

If you have enabled the Administrator account and tried to go to the Windows Store from within that account, it will fail with an error “…cannot access Store from Administrator account..blah blah.”.

Therefore you will need to create a new Temp user account, give it local administrator privilages, and run the update from within that Temp account.

If the upgrade does NOT even show up in the Windows Store:

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!