NixOS: Setting the default channel in configuration.nix

system.autoUpgrade.channel is what you might be looking for

set it to any channel e.g.

system.autoUpgrade.channel = "https://nixos.org/channels/nixos-16.03-small/";

the documentation says:

by default, this is the channel set using nix-channel (run nix-channel --list to see the current value)

an up to date list of channels can be found on https://nixos.org/channels/

ref: https://nixos.org/nixos/manual/options.html#opt-system.autoUpgrade.channel https://nixos.org/nixos/manual/index.html#idm140737317454064

Set nixPath = [ "nixpkgs=http://nixos.org/channels/nixos-unstable/nixexprs.tar.xz" ];, see https://github.com/snabblab/snabblab-nixos/blob/d8b9761b107293891b19021f2f0f77a0e3ba3746/modules/common.nix#L39