TodoWP 2.0 has not been released yet, there is only a 2.0 beta available for developers and testing.
If you are running a forum in production, don't update yet.
If you are running a test forum, it's probably a better idea to create a fresh installation of the Flarum beta.
If you really need to perform an upgrade to the beta, take a look at the older migration guides, I don't think anything really changes. Even there it's simpler to do a fresh install. Disable all third-party extensions, make database and assets backup, create fresh install, copy database and assets, then re-install extensions. Run migrations and clear cache each time.
