As of NixOS 19.03 (Koi), you can open the system configuration in the default system editor with the following command:

$ sudo nixos-rebuild edit

Alternatively, you can edit the file located at /etc/nixos/configuration.nix

In order to apply any changes to the configuration.nix file, the NixOS configuration needs to be rebuilt.

The nixos-rebuild command is used to rebuild the system configuration. In general, there are two commands you may want to run in order to properly build the system:

$ sudo nixos-rebuild switch

Running this command performs the following:

• Rebuilds the current configuration.nix file
• Downloads any new packages
• Adds a new entry to the boot menu
• Applies the changes of the configuration right away (whilst you're still using the system. It's basically seamless!)

Any new packages can be used right away, any new services will be started right away, however environmental variables will not be activated until the next reboot.

$ sudo nixos-rebuild boot

This basically does everything that sudo nixos-rebuild switch does, except it does not apply the changes of the configuration right away. When the system is rebooted, the newly built configuration will be used.