The new Windows 8.1 will be released today as a preview. The final version of Windows 8.1 will be released in Fall.
Microsoft will make Windows 8.1 preview release available to anyone starting today. If you want to get your beloved start button back, today is the day. The final version of Windows 8.1 (aka Windows Blue) comes sometime late this year.
Microsoft offers Windows 8.1 in the Windows Store as an app update and as an ISO image to create a dual boot setup. According to Softpedia, the ISO download link will be http://go.microsoft.com/fwlink/p/?LinkId=301772. This Windows 8.1 download link is not live yet though.
New features in Windows 8.1 include Boot to desktop, Improved IE, Device Lockdown, Work Folders, Workplace Join, Open MDM, Mobile Device Management, Web Application Proxy, RDS Enhancements, NFC Tap-to-pair Printing, Wi-Fi Direct Printing, native Miracast and many more. You can find out more about what is new in Windows 8.1 in this overview on Microsoft.