To rename the main theme follow these easy steps:
Renaming your theme will stop its automatic updates, you should do it manually in the future.
- Go to /wp-content/themes/YOUR_THEME and rename it to whatever you want (e.g. NEW)
- Go to the theme folder /wp-content/themes/NEW and open the style.css file. In the top part of the style.css file you will see the theme name. Rename it and save changes
- Finally, navigate to Appearance → Themes from your WordPress admin panel and activate your theme under the new name
If you want to use both parent and child themes with a new name you should make an additional step:
- Go to the child theme folder /wp-content/themes/NEW-child and open the style.css file
- Change the parent theme path in the style.css file and save changes