On windows, it keeps saying and starting v3.5 instead of the latest and I can't see how to get the new one. It's a great game, but I'd rather not have to restart if there is a way around this.
You can safely delete the game folder, download and unzip the new version.
Your save files are stored in the AppData folder, not in the game folder, and will be detected automatically.
However, I'm not sure if the save file will become corrupted if you start from such an old version.