Windows 8.1 Update is available to download for the MSDN and TechNet subscribers. If you are not an MSDN or TechNet subscriber it is expected to release tomorrow April 8, 2014.
The Windows 8.1 Update delivers a collection of refinements designed to give people a more familiar and convenient experience across touch, keyboard and mouse inputs. It also brings improvements for business customers, really accelerates opportunity for developers, and enables device makers to offer lower cost devices.
Watch the video below to check what is new on Windows 8.1 Update
The most interesting features are
Access the taskbar from anywhere when moving mouse to the bottom of the screen.
Pin App store apps to taskbar
Close and minimize buttons on App store apps
Read more here