Microsoft has decided to release the first update to Windows 8.1 on April 8th through Windows Update. The ISO files of 'Windows 8.1 Update' have already been uploaded on Microsoft's server, and MSDN / TechNet customers can download them right now.
For those who don't know, Windows 8.1 Update offers a lot of tweaks to the OS for the desktop users who use mouse and keyboard to control their PCs. For example, a new title bar will be displayed at the top of the modern apps, making it possible to minimize and close the apps with a mouse. Apps can now be pinned to the taskbar and the taskbar will be accessible when you will be using the modern apps. You will have to move your cursor to the bottom of the screen, and then push down slightly to make the taskbar appear. A new shutdown button will also be there on the Start Screen, alongside a search button.
Source: Paul Thurrott