Windows 8.1 Update 1 has been leaked to various file sharing websites ahead of its March release. The leaked version was compiled on January 14th and the full build string of the leaked ISO is 9600.16596.WINBLUES14_GDR_LEAN.140114-0237. This build is compatible with 64-bit computers only.
Please keep in mind that It's an early build, compiled on January 14th and the bugs could affect the overall performance on your computer so it's will be a wise decision to wait for the official release in March 2014.
Windows 8.1 Update 1 will bring some new features like titlebar for Modern apps which will make it easier to minimize or close them, ability to pin apps to the taskbar, thumbnail previews for the apps etc. It will also reduce the memory and disk space requirements for Windows 8.1.
