The most basic method is to use the psql tool. The \password command will prompt you once for a new password and again to confirm. Connect to the psql tool and type the following:
\password
Enter a new password. This causes psql to send an SQL statement to the PostgreSQL server, which contains an already encrypted password string. An example of the SQL statement sent is as follows:
ALTER USER postgres PASSWORD 'md53175bce1d3201d16594cebf9d7eb3f9d';
Whatever you do, don't use postgres as your password. This will make you vulnerable to idle hackers, so make it a little more difficult than that!
Make sure you don't forget your password either. It may prove difficult to maintain your database if you can't get access to it later.