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1st chunk of `nixos/doc/manual/installation/upgrading.chapter.md`
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# Upgrading NixOS {#sec-upgrading}

The best way to keep your NixOS installation up to date is to use one of
the NixOS *channels*. A channel is a Nix mechanism for distributing Nix
expressions and associated binaries. The NixOS channels are updated
automatically from NixOS's Git repository after certain tests have
passed and a selection of packages has been built successfully
(see `nixos/release-combined.nix` and `nixos/release-small.nix`).
These channels are:

-   *Stable channels*, such as [`nixos-25.05`](https://channels.nixos.org/nixos-25.05).
    These only get conservative bug fixes and package upgrades. For
    instance, a channel update may cause the Linux kernel on your system
    to be upgraded from 4.19.34 to 4.19.38 (a minor bug fix), but not
    from 4.19.x to 4.20.x (a major change that has the potential to break things).
    Stable channels are generally maintained until the next stable
    branch is created.

-   The *unstable channel*, [`nixos-unstable`](https://channels.nixos.org/nixos-unstable).
    This corresponds to NixOS's main development branch, and may thus see
    radical changes between channel updates. It's not recommended for
    production systems.

-   *Small channels*, such as [`nixos-25.05-small`](https://channels.nixos.org/nixos-25.05-small)
    or [`nixos-unstable-small`](https://channels.nixos.org/nixos-unstable-small).
    These are identical to the stable and unstable channels described above,
    except that they contain fewer binary packages. This means they get updated
    faster than the regular channels (for instance, when a critical security patch
    is committed to NixOS's source tree), but may require more packages to be
    built from source than usual. They're mostly intended for server environments
    and as such contain few GUI applications.

To see what channels are available, go to <https://channels.nixos.org>.
(Note that the URIs of the various channels redirect to a directory that
contains the channel's latest version and includes ISO images and
VirtualBox appliances.) Please note that during the release process,
channels that are not yet released will be present here as well. See the
Getting NixOS page <https://nixos.org/download/> to find the newest
supported stable release.

When you first install NixOS, you're automatically subscribed to the
NixOS channel that corresponds to your installation source. For
instance, if you installed from a 25.05 ISO, you will be subscribed to
Title: Upgrading NixOS with Channels
Summary
The recommended way to keep NixOS up-to-date is through NixOS channels. Channels are Nix mechanisms for distributing Nix expressions and binaries. There are stable channels (like `nixos-25.05`) that receive conservative updates, an unstable channel (`nixos-unstable`) for the main development branch, and small channels (like `nixos-25.05-small`) with fewer pre-built binaries. The channels available are listed at https://channels.nixos.org. New installations are automatically subscribed to the channel matching the installation source.