Although the Windows 8.1 update is downloaded and installed from the Store,in some cases 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 2871389 manually.
Or you can download the package from the link below and install:
If you still don't see the update on the Store home screen after installing the KB 2871389 update, copy and open this link on your browser: ms-windows-store:WindowsUpgrade