In the rare case you’re still running the Windows 8.1 Preview it expires in less than a month. Windows 8.1 is a free upgrade so need to wait. Here’s what to do:
- If you installed the Windows 8.1 Preview or Windows RT 8.1 Preview from the Windows Store, you can update to the final release through the Windows Store by following the instructions available here, or by clicking here to initiate the process. Note that you will have to reinstall all of your applications (desktop or modern) after doing this.
- If you installed a volume license version of Windows 8.1 Preview for evaluation purposes, we would recommend that you install a fresh version of Windows 8.1 Pro or Windows 8.1 Enterprise for ongoing use. If you have to upgrade an existing machine, you can do so using volume license media. This can be done by manually running SETUP.EXE from the media, or can be automated by running “setup.exe /auto:data”. As with any Preview update, only data and accounts will be migrated; all apps will need to be reinstalled.
Microsoft has a detailed blog post for IT Pros: Windows 8.1 General Availability: The IT Pro Perspective.
Source: Springboard Series