Originally posted by Zep Tepi:

Go to the side menu in your library, select betas and default_old. That will put it back to before the update.

I did this for my mods, but I'm definitely not switching back until the lefty mouse problem is fixed.

Added: They said they fixed it on the test server, so it should be on live soon.

OP said they play on server so no roll back, btw have the same problem