module replacement example

configuration.nix

{ config, pkgs, ... }:

let

  # Import unstable channel.
  # sudo nix-channel --add http://nixos.org/channels/nixos-unstable nixos-unstable
  unstable = import <nixos-unstable> {};

in

{
  disabledModules = [ "services/databases/postgresql.nix" ];

  imports =
    [ # Include service from unstable channel.
      <nixos-unstable/nixos/modules/services/databases/postgresql.nix>
    ];
  
  services.postgresql.enable = true;
  services.postgresql.packages = unstable.postgresql;
}

Thanks! :)