You cannot join official servers or play with friends running newer versions. This is strictly for single-player or private, version-matched servers.

Since Steam automatically updates games, you cannot simply select "v0.1.5.1" from a dropdown menu. You must use the to "downgrade" your files. Step 1: Open the Steam Console Press Win + R on your keyboard. Type steam://open/console and hit Enter. A new Console tab will appear in your Steam client. Step 2: Use the Download Command