Open terminal and run following commands:
cd your_app_dir heroku apps:rename [new_name] --remote [remote_name]
OR
cd your_app_dir heroku apps:rename [new_name] --app [oldname]
Open terminal and run following commands:
cd your_app_dir heroku apps:rename [new_name] --remote [remote_name]
OR
cd your_app_dir heroku apps:rename [new_name] --app [oldname]