Alpha 19 is now officially out, so it is no longer found in the Betas tab. If you've selected something in the Betas tab, please set it to "NONE - opt out of all beta programs."
Steam will then automatically download the update. If it's not, then it may make sense to try restarting Steam again.
![Smiley :)](http://universesandbox.com/forum/Smileys/default/smiley.gif)
Hope that helps.