1. First you need to login heroku:

heroku login

2. Then you need to download the code:

heroku git:clone -a <project_name>

Get a remote repo:

 git remote add heroku git@heroku.com:<project_name>.git

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

7. Restart the heroku

heroku restart -a <project_name>