In some cases, the developers might ask you to try a different distribution branch, for example, to test if it solves bugs, etc. The procedure to switch to a specific branch (named "beta" in this example) is as follows:
To select a branch in Steam
Right click on the game name and select Properties.
Click the BETAS tab (1)
Enter the access code or password provided by the developer (2)
Click CHECK CODE (3)
Select the branch from the list, "beta" in the example (4)
Click CLOSE (5)
Wait for the download.
The name of the game will have the branch name between brackets ("[beta]" in this example).
To return to the default game version,
Perform the same procedure selecting NONE from the list in (4).