How to update (and whether or not you should)
_tw does not have versions, and there is no expectation of backwards compatibility between the most recent version and ones from the past. This philosophy matches that of _s, as outlined in the post, Don’t Update Your Theme (From _s).
In summary, once you generate your theme, you shouldn’t worry about keeping it in sync with every change from _tw.
Nonetheless, you may want to update to the latest version of Tailwind within your previously generated theme. It’s generally safe to use npm update, though you’ll always want to make sure you have a past commit you can go back to in case things go wrong. Alternatively, you could copy the most recent devDependencies from package.json and the most recent package-lock.json and then run npm install.
But you may not find even updates of this sort to be necessary unless you specifically need a new Tailwind feature.
If there are bugs in _tw, or if _tw’s integration with WordPress or another dependency breaks for any reason, you will find commits specific to fixing those bugs on GitHub alongside a list of steps to fix the bug in an existing theme, usually within the corresponding issue for that bug.