How to set password in postgres sql?

Question

PostgreSQL

I do everything according to the guide when changing the user sudo su - sergey asks for a password for the postgres user. So, I went to the psql console and changed the password for the postgres user
ALTER USER postgres PASSWORD 'password'; but when changing sudo su - sergey again asks for a password and I enter it, and he says it's wrong. what should I do so that I can switch to user sergey???