Switch to the heroku app repository which name you want to change.
heroku apps:rename your-new-appname(Name must start with a letter and can only contain lowercase letters, numbers, and dashes.)
heroku opento open your app.
You will see your url become
https://your-new-appname.herokuapp.com/on your web browser.
Notice! After change the appname, your old app url will become unavailable.