rename heroku app

First update the name for the heroku app on the heroku dashboard (on the heroku website)
Then do this on terminal (Make sure you're in the right repository)

$ git remote rm heroku (Remove the remote from git)
$ heroku git:remote -a <new app name> (Add it back with it's new name)

$ git remote rm heroku
$ heroku git:remote -a newname

$ heroku apps:rename newname
Renaming oldname to newname... done
http://newname.herokuapp.com/ | [email protected]:newname.git
Git remote heroku updated