Hi Milos,
Betas are available to Steam users once they reach the "release candidate" stage. We're at that point now, so I believe you should be able to get 10.50r1 on Steam.
To try the release candidate, you’ll need to go to the “properties” of X-Plane (in the Steam client) and opt in to betas. (If you can’t figure out how to do this, you should not opt in to betas!)