I can give you code to remove the GeneratePress page from the admin, and remove the options from the Customizer, but if you want to change everything, including the folder name etc, you’ll have to manually change the files.
1. Go to wp-content/themes and change “generatepress” to your own name. Note: This will stop you from receiving automatic updates.
2. Go to wp-content/themes/your-new-name and open style.css – change the name of the theme in there.
3. Use these functions to hide GeneratePress in the Dashboard: https://gist.github.com/generatepress/f04d0881f806e84be0ac