As explained in the manual’s section on upgrading, if you issue the following command as root:
# nix-channel --list
you will most likely see something like the following (if you were following the 16.09 branch for instance):
nixos https://nixos.org/channels/nixos-16.09
By issuing the following command (still as root):
# nix-channel --add https://nixos.org/channels/nixos-unstable nixos
you will remove the 16.09 channel and replace it with nixos-unstable.
Now this is not enough yet. To really update your system, you need one last command:
# nixos-rebuild switch --upgrade
which is going to rebuild your system with your current configuration and updated packages from the channel you switched to.