Updating your WordPress version is just a matter of merging the updates into the branch created from the installation.
$ git pull upstream # Get the latest
Using the same branch name from our installation:
$ git checkout production
$ git merge master # Merge latest
$ git push heroku production:master
WordPress needs to update the database. After push, navigate to:
http://your-app-url.herokuapp.com/wp-admin
WordPress will prompt for updating the database. After that you'll be good to go.