I've heard from several people where the Windows 8.1 RT upgrade is still not available to them in the Windows Store app even though Microsoft reposted the RT bits on October 22, 2013. This is usually due to caching issues with the app. In fact, even in Windows 8.1, the Windows Store app still seems to rely too much on caching, as it takes days before the new apps list is refreshed.
So, if you still can't locate the Windows 8.1 upgrade, even after searching for days, here's a simple way to get access to the Windows 8.1 upgrade page in the Windows Store app.
In the "metro" side of Windows 8, open Internet Explorer and type in and run the following command link:
ms-windows-store:WindowsUpgrade
This will take you directly to the Windows 8.1 upgrade page.
Alternatively, you can also run this same command in the desktop side of Windows 8, but you have to run it from the "Run" command instead of the desktop version of Internet Explorer.