There’s a couple options – none of which will lose any of your settings.
a) Go to “Appearance > Editor” – after the description, you’ll see: Version: 1.3.15
Change that to: Version: 1.3.14
That will trick WP into thinking you have the old version, and it will tell you there’s an update available.
b) Go to “Appearance > Themes” and activate a Twenty-something theme. Then delete GeneratePress, install the fresh version and activate it.
We actually got really lucky we only lost some javascript files – other themes lost core PHP files which resulted in a lot of websites crashing.
Hopefully WordPress.org has found what caused the issue and will stop it from happening again.