Seasons major update is now available on Beta branch. Try it out!
To switch to a custom branch, right-click on the game from the library page and select "Properties". Among the available tabs will be the "Betas" tab. Open it and select "beta - public betas, unstable builds etc" in the dropdown on the right.
You can start a new game, but it should also work fine with existing saves from any older versions (just be aware you won't have control over the snowlines setup then). Please note that beta versions might be unstable, containing bugs or other issues. If you notice anything (or just want to share your feedback about the changes) please report it through the usual channels (Steam discussions, our Discord server or directly from the game using the "Send feedback" button).