personally i would always move to a Staging site when making changes to a live site. Just in case something goes a little wrong 🙂
If you do it on the live or the staging site then you can do the following:
1. Appearance > GeneratePress –> Reset Settings.
2. Appearance > Customizer > Additional CSS –> Delete any of the CSS imported by the site.
3. Appearance > Elements –> Delete any elements imported by the Site.