The updates will be applied automatically when your Steam client is restarted.
The major changes include:

* New Progress UI implemented (PC/Mac)
* Updated to Bullet 2.77 (PC/Mac)
* A lot of engine optimizations (PC/Mac)
* Several memory leaks fixed (PC/Mac)
* Stability issues in LUA scripting fixed (PC/Mac)
* Reduced memory needed to load SWF (PC/Mac)
* Added support for HID joystick devices (ie Logitech gamepads). (Mac)
* Fixed enumeration of interlaced and/or TV-output modes in OpenGL. (Mac)