The replication system administrator can change any user's password with the alter user command. A user can change his or her own password.
alter user user set password {new_passwd | null} [verify password old_passwd]
The same rules that you use for specifying the password using create user also apply to alter user.
The verify password clause, which prevents users from altering each other’s passwords, is required for users without sa permission.
alter user louise set password polyESter verify password null