From Valve Developer Community
Steam often offers the opportunity to try out game updates or Steam updates before they're officially released.
Joining a Beta test
To join one of these beta programs:
- Go to Steam > Settings in the Steam client. The Beta Participation section will tell you if there are any beta programs available, or if you are currently running one of them.
- Click on the Change... button in the client participation section and select one of the choices available in the drop-list of beta programs.
- Click OK and restart Steam when you are prompted to do so.
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).You may only select one beta program at a time.
To opt out of a beta, follow the installation instructions and select NONE - Opt out of all beta programs from the list.
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 restart Steam). If you encounter any bugs, please report them to us according to the guidelines on the Beta page for that particular program.
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.