Windows 8 Upgrade to Windows 8.1: Can not find the update in the Store? Or Error

As per official MS response:

"Already running Windows 8 Pro? No need to buy Windows 8.1 Pro—you can update to Windows 8.1 Pro for free through the Windows Store."

Reply: http://windows.microsoft.com/en-us/windows-8/why-can-t-find-update-store

Links:
http://support.microsoft.com/kb/2917499
http://support.microsoft.com/kb/2871389

* ...and enough free disk space if your are upgrading (or you will get a notification to have 6.9GB free on your C:\)