steam – How do I set what beta branch I want to upload my build to in Steamworks?

The documentation for the setlive parameters say:

Branch name to automatically set live after successful build, none if empty.
Note that the ‘default’ branch can not be set live automatically. That must be done through the App Admin panel.

You cannot accidentally set the default branch live, if that’s what you are worrying about. If you are worried about accidentally setting another beta-branch live while testing, just create another password protected branch for testing this before deploying it into your proper build pipeline.