Source claims that some Microsoft partners are now seeing the update in Windows Store
The official Windows 8.1 Preview will see daylight on June 26, but it appears that some lucky Microsoft partners are already getting the update via the Windows Store.According to a report published by Win8China, select Microsoft testers can now download the preview from the Store, but it's not yet clear if this is another early build or the version that the company wants to debut at BUILD.
A screenshot also included in the article indicates that only some Chinese users are now seeing the update, so chances are that you won't be able to download it until June 26.
Keep in mind that this is an unconfirmed report, so take everything with a pinch of salt. Windows 8.1 Preview will go live later this month, so that's the time when everybody should download the Windows 8 update from the Store.