Well, tbh this question is kinda useless, because the error message says it all. You have to free up space on your hard drive, 80GB to be safe.
Just say yes.
It won't install the game again, it will just install the new installer.
(for developers: obviously there should be some sort of check to see if the game is actually already installed, but obv this use case - people with the game needing to update only the installer - was never considered when developing/updating the installer. This is poor.)
Updating it was a fairly painless experience for me. Hopefully it's the same for everyone.
Now we can get back to having fun in game