1. Open terminal.

  2. Switch to the heroku app repository which name you want to change.

  3. Type: heroku apps:rename your-new-appname (Name must start with a letter and can only contain lowercase letters, numbers, and dashes.)

  4. Then type: heroku open to open your app.

  5. You will see your url become https://your-new-appname.herokuapp.com/ on your web browser.

  6. Done!

Notice! After change the appname, your old app url will become unavailable.