This short guide will explain you how to switch a NixOS installation to the unstable channel, understand the development version.
nix-channel --add https://channels.nixos.org/nixos-unstable nixos
You will have to reload the channel list using the command "nix-channel --update" and then you can upgrade your system using "nixos-rebuild switch".
If you have issues, you can rollback using "nix-channel --rollback" that will set the channels list to the last state before "--update".