It’s updated whenever your package manager puts up the new version. If you’re using something like Ubuntu or Mint, then you’re using apt as your package manager, which takes a while to update stuff because they like to test it for a while first.
Its supposed to release early next year but if you’re on the stable channel it might be a little bit more of a wait. Someone can correct me if I’m wrong but when its released and you run your system update, cinnamon will be updated.