how do i change the password for 'postgres' user - Mailing list pgsql-admin

From Gopi G
Subject how do i change the password for 'postgres' user
Date
Msg-id CAC9HVwmbzYXdPixFzRwmEVe0XJhw5OTtxHrv2BcTEM3UDX77=g@mail.gmail.com
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Responses Re: how do i change the password for 'postgres' user  (Francis Santiago <santiagocfc@gmail.com>)
Re: how do i change the password for 'postgres' user  ("David G. Johnston" <david.g.johnston@gmail.com>)
List pgsql-admin
I have postgres 9.2 running in AWS EC2 (aws linux) instance. 
I followed documentation and tried to do 

postgres=> ALTER USER Postgres WITH PASSWORD 'password';

ERROR:  must be superuser to alter replication users


surprisingly I am not finding any way to solve this problem. 
and how do I login using 'postgres' linux os user that is created when I installed postgres 9.2?

gopi

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