Betas
Steam often offers the opportunity to try out game updates or Steam updates before they're officially released.
To join one of these beta programs, launch Steam and open the "Settings" page (File->Settings, or click the "Settings" button). The "Beta Participation" section will tell you if there are any beta programs available, or if you are currently running one of them. If there are betas in progress, you can click the "Change" button to select one.
To opt out of a beta, follow the above instructions and select "None" from the list.
You may only select one beta program at a time.
After changing your Beta selection, you'll be asked to restart the Steam client. If you selected a Steam client beta, then your Steam client will update itself to the Beta version at this point. If you selected a game beta, then Steam will restart and begin updating that game (if you have automatic updates enabled for that game; if not, it will update it the next time you run the game).
As the beta progresses, we may release further updates for it. Steam should apply these updates for you automatically, whether it is a Steam client beta (it will ask you to restart), or a game beta (they'll be applied the next time you run the game or resatart Steam). If you encounter any bugs, please report them to us.
When the beta you have selected ends, Steam will let you know, and automatically clear the selection for you, then ask you to restart the Steam client.
If you encounter a problem with the beta and are unable opt-out of it using the Settings dialog, you can clear the selection by deleting all the values in HKEY_CURRENT_USER\Software\Valve\Steam\Beta in the Windows Registry.