From the
FAQ on the Steam forum:
A) Builds
Q1: How can I play the latest experimental build?A: Before you read the answer, understand that experimental builds are bleeding edge.
Unstable. There will be bugs. It may be unplayable. You may lose your saved game or your world. That’s why it’s not released to everyone, and you have to voluntarily opt in. Still feeling brave? Okay. Right click on the game in your Steam library and navigate to Properties... -> Betas -> latest_experimental. Then Steam will download the experimental build. Note that there is not always an experimental build available.
Q2: How can I reject future updates and stay on this build, or play an older build of the game?A: Right click on the game in your Steam library and navigate to Properties... -> Betas. Whatever build you select here is the build you’ll download and play, until you change it.
Q3: Do I have to start over with the new Alpha? Can I still play my old saved games? Will they be deleted?A: Your saved games are only expected to work with the Alpha you were playing when you started them. If you don’t want to start over, you can roll back to that earlier Alpha to play your old saved games - see previous question. An update will not delete your saved games.
Q4: I want to play the latest stable build, but I don’t see it in the betas tab with the other versions.A: The latest stable build is what users download by default
instead of a beta build. Select “None” from the betas tab to download it. Note that you may have seen the
same build with the
same build number on the betas tab in the past, because the build that is now stable, used to be experimental.
Also, remember that if you do nothing and don’t manually opt in to A20, you’ll stay on A19 during the experimental phase. You won’t download anything until later, when A20 goes from experimental to stable.