How to Access files on Heroku?
You can run heroku run bash and backup the file then deploy your app run heroku run bash again and remove the new file and replace it with the old one you backed up. Hope it helps
I have a project on Heroku. This project allows me to update a JSON file. If update something on Heroku via the project's web interface I see the update. I can close the browser, open it again and the update persists. 
Now, I want to push something to this project. If I do, the JSON file will be overwritten. So, I pulled first to get the current project state from heroku. Heroku's prompt says that everything is up to date, which is not true since I changed the JSON file. 
Using heroku run ls static/json/ shows the changed file. I need to get this file before pushing to heroku to avoid destroying updates done via the web interface.