1. First you need to login heroku:

heroku login

2. Then you need to download the code:

heroku git:clone -a <project_name>

3. After you done your changes, cd to project root folder:

cd <project_name>

4. State the changes:

git add --all

5. Commit the changes:

git commit -am "make it better"

6. Pust the changes:

git push heroku master

[Heroku] How to pull, push changes