Owners of Windows 8 computers and devices in 230 markets can now download 8.1, which brings back the popular Windows Start button.
Microsoft abandoned the start button with the first live-tile interface in the Windows 8 operating system, which was introduced in October 2012.
Windows 8.1 updates also include a customizable Start screen in live-tile mode as well as the old Windows desktop version. The update will also allow syncing of all Windows devices to a Microsoft user account and introduces upgraded Bing Smart Search.
The Windows update also offers native support for 3D printing.
"With Windows 8, we introduced an era of highly mobile, touch-enabled, and always-connected computing. Windows 8.1 evolves this vision and brings together everything people do across all their devices whether at home, at work, or on the go," Microsoft said in a statement.
The company is also making multitasking a major selling point, noting the system's ability to "display and use up to four apps side by side, flexible windows sizing for each app on the screen, the ability for one app to launch another app, and improved multi-monitor support so users can view the desktop or apps from the Windows Store on any or all monitors."
As Window 8.1 rolls out, dozens of apps have released their updates, including Mail, Calendar and People. The Xbox Music App and Xbox SmartGlass have also updated. Third party updates were led by Facebook, Skype and Netflix.