How do I migrate/change database?

[sebzapata]

Hi,
I currently have my app set up and deployed via Heroku which was done using the "Running on Heroku" guide in the docs.
I am just about to exceed the 10,000 rows limit of my Postgres DB and would like to change the DB to one with a larger limit from Elephant SQL.
How would I change the database that is written/read from? I assumed I would just need to update the DATABASE_URL in the docker-compose.yml file, but this doesn't seem to have done what I expected.

Thank you

[mikecao]

All configuration is done in the .env file.