Multi-user Nix users on macOS can upgrade Nix by running sudo -i sh -c 'nix-channel --update && nix-env -iA nixpkgs.nix'; sudo launchctl stop org.nixos.nix-daemon; sudo launchctl start org.nixos.nix-daemon. Single-user installations of Nix should run nix-channel --update; nix-env -iA nixpkgs.nix.