I want to change the timezone to 'Brazil / East' for Postgres.
We highly suggest leaving your Database times in UTC and modifying them at your application level. This results in your data being more ubiquitous and easier to convert when needed. Having dates in the database using a different timezone to be problematic since many libraries and functions often expect dates and times in UTC.
If you need to change it, you can do so by connection to your Postgres instance with
$ heroku pg:psql and running
ALTER database your_database_name SET timezone ='Brazil / East' but please consider the repercussions of changing the timezone for your application.