All installs are set to the stable version of the game by default.
To change this:
In your Steam client, go to the Library and right (mouse button) click on Rust
Select Properties (from context menu)
Select the Betas tab
Choose either NONE for the Stable version, Staging or Prerelease
Click Close in the lower-right corner of the window
Steam will start automatically downloading the latest version
Using the development version of the game could hamper your experience while playing,
with some unfinished assets, features and bugs - Expect servers to wipe at anytime.
However, we would greatly appreciate your feedback.
More details on branches and how we use them can be found here.