'Underscores' (_s) isn't a theme. Not really. It's the starting point for a new theme. When you used it to create a theme you just created a brand new theme using Underscores as a head start.
I'm a theme meant for hacking so don't use me as a Parent Theme.
Instead try turning me into the next, most awesome, WordPress theme
out there. That's what I'm here for.
So no, you're not supposed to update Underscores. There isn't really even anything to update. At this point it has become your theme and doesn't exist on its own.
But as you say, Automattic does update Underscores. They would do this to make improvements & tweaks, and add features, but these updates are intended to improve future projects based on Underscores, not exiting Underscores based themes.
If you see an update to Underscores that you would like to implement on your own Underscores based theme, then you will need to inspect the change and figure out how to manually implement it on your own theme, and you'll need to approach these changes case by case.