Usually this is done in the Steam library.
Right click on the game guru icon and choose properties.
Choose the beta tab and see what is available.
The Christmas build should be there and that should be the latest version out for Beta.
Keep an eye out there.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.