This afternoon Apple released the third update to OS X Mavericks for Macintosh. Apart from the normal bug fixes and security fixes, this update brings new features that don’t affect most people, such as support for new 4K monitors and restoring the ability to sync your iOS devices by USB instead of by iCloud.
To update click the Apple menu in the top left corner of the screen and choose “Software Update”. After a few moments you should see “OS X Update 10.9.3” in the list of available updates. Click the “Update” or “Update All” button to the right.