postgres: upgrade a user to be a superuser?

To expand on the above and make a quick reference:

• To make a user a SuperUser: ALTER USER username WITH SUPERUSER;
• To make a user no longer a SuperUser: ALTER USER username WITH NOSUPERUSER;
• To just allow the user to create a database: ALTER USER username CREATEDB;

You can also use CREATEROLE and CREATEUSER to allow a user privileges without making them a superuser.

Documentation

$ su - postgres
$ psql
$ \du; for see the user on db
select the user that do you want be superuser and:
$ ALTER USER "user" with superuser;

ALTER USER myuser WITH SUPERUSER;

You can read more at the Documentation