Microsoft is schedule to be releasing the Windows 8.1 Preview on June 26during the Build conference and we know that the update would be offered for free via Windows Store for all Windows 8 users. But, we didn’t know the details of how the update would be available and what will happen, until now.
Windows 8,1 Preview on Surface

ZDNet reports that Microsoft during the TechEd talked about the steps of getting the Windows 8.1 Preview on Windows 8 when it is released later this month. Users will get a prompt notification via Windows Updates that the Preview is available.
The notification will take you to Windows Store where the details of the Windows 8.1 Preview would be offered and users will have the option to install or deny to install the Windows 8.1 Preview. In the last, Windows 8.1 Preview would be downloaded from Windows Store.
Microsoft would also be opting the same method for the final RTM (release to manufacturing) version of Windows 8.1 which would be released later this year.
You can watch the preview of Windows 8.1 in a video by Microsoft here.
Update:
Here is how the notification will look like which you will receive in Windows 8. Thanks to @sM4llziE on Twitter.
Windows 8.1 update notification in Windows 8
Source: ZDNet