nixpkgs/nixos/doc/manual/administration/rollback.xml

<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-rollback">
 <title>Rolling Back Configuration Changes</title>

 <para>
  After running <command>nixos-rebuild</command> to switch to a new
  configuration, you may find that the new configuration doesn’t work very
  well. In that case, there are several ways to return to a previous
  configuration.
 </para>

 <para>
  First, the GRUB boot manager allows you to boot into any previous
  configuration that hasn’t been garbage-collected. These configurations can
  be found under the GRUB submenu “NixOS - All configurations”. This is
  especially useful if the new configuration fails to boot. After the system
  has booted, you can make the selected configuration the default for
  subsequent boots:
<screen>
# /run/current-system/bin/switch-to-configuration boot</screen>
 </para>

 <para>
  Second, you can switch to the previous configuration in a running system:
<screen>
# nixos-rebuild switch --rollback</screen>
  This is equivalent to running:
<screen>
# /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>
  where <replaceable>N</replaceable> is the number of the NixOS system
  configuration. To get a list of the available configurations, do:
<screen>
$ ls -l /nix/var/nix/profiles/system-*-link
<replaceable>...</replaceable>
lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
</screen>
 </para>
</section>