resetting superuser password

I have lost the superuser (user postgres) password, but I still have
the unix root password. Is there anything I can do to reset the
postgres user's password (short of reinstalling Postgres)?

Obviously, I can su to unix user postgres, but this does not seem to
advance my cause, as doing "psql -U postgres" still prompts me for a
password, which is not the same as the unix postgres user's password.

OS: RedHat 9

Help!

Thanks,
Marc