Step 1: Firstly go to wp-content/themes/ folder. And then rename your theme folder to whatever you want.
Step 2: Open your theme folder and open style.css file. In top part of style.css you will see theme name. Rename it and save changes.
Step 3: Go to Wp-admin/appearance/themes and activate your theme under new name.
If you are using child/parent theme and you also rename parent theme folder&name, so after Step 3 you should additionally change parent theme path (template field) in child theme’s style.css.
note : renaming your theme will stop its automatic updates, you should do it manually in the future.