Experimental flakes support is required. https://www.tweag.io/blog/2020-07-31-nixos-flakes/
sudo nixos-rebuild switch --flake .#hostname
#host-name is optional sudo nixos-rebuild --flake . will use the current name of the host.
Note: untracked files won't be copied to store. Use git add first, to build from a dirty directory.
To update nixpkgs input to latest unstable:
nix flake update --update-input nixpkgs