You type nixos-rebuild switch and it builds you an entire OS and drops you into it. It also makes the new OS the default on startup. Because it's all pure

You type nixos-rebuild switch and it builds you an entire OS and drops you into it. It also makes the new OS the default on startup. Because it's all purely declarative, your old OS is still there. If things go wrong then you can just pop back to it, either in the command line with: nixos-rebuild switch --rollback Or when you reboot, the previous 'generations' of your operating system show up in GRUB, so just choose an older one to boot.