How to change the default user for psql command

When I run the "heroku pg: psql" command, it tries to authenticate me under "justinwong". This is a problem because it is not a user in the list of roles when I run the \ du command in postgres.

The only user that exists should be postgres.

How can I change the default user so that the "heroku pg: psql" or "psql" command will try to log in so that it runs postgres and not justinwong?