how do i change the password for 'postgres' user

[Gopi G <gopiputty@xxxxxxxxx>]

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