Every time I make a small update to an app I've already deployed, I find myself going back to the Heroku documentation. Here's a quick guide on how to update an app that you've already deployed.
git clone <YOUR HTTPS URL FROM GITHUB>
git add .
git commit -m "<YOUR COMMIT MESSAGE>"
git push origin <YOUR BRANCH NAME>
Follow the directions on the screen to login to your account through the browser, then return to the terminal.
heroku git:remote -a <YOUR PROJECT NAME>
git push heroku master
And that's it! 5 simple steps. Hopefully this might save you from digging through the documentation each time!