To access the legacy version of THOTH which was default until June 2017 simply:

1. Go to your Steam library
2. Right click on THOTH and choose "Properties"
3. Under the "BETAS" tab, select "legacy - 2016 version"

You might have to re-verify the game cache and/or restart Steam.

Congratulations! You can now access lava levels and hate yourself. :)