This was the first result on google, when I was looking how to rename a user, so:
ALTER USER <username> WITH PASSWORD '<new_password>'; -- change password
ALTER USER <old_username> RENAME TO <new_username>; -- rename user
A couple of other commands helpful for user management:
CREATE USER <username> PASSWORD '<password>' IN GROUP <group>;
DROP USER <username>;
Move user to another group
ALTER GROUP <old_group> DROP USER <username>;
ALTER GROUP <new_group> ADD USER <username>;