docs: add upgrade guide for NixOS version transitions

- Add comprehensive documentation for upgrading Home Manager channels.

- Cover channel-based, flake-based, and NixOS module methods.

- Include troubleshooting section and state version updates.

- Update examples for NixOS 25.05 compatibility.

docs/manual/usage.md

@@ -61,4 +61,5 @@ usage/dotfiles.md
 usage/graphical.md
 usage/gpu-non-nixos.md
 usage/updating.md
+usage/upgrading.md
 ```

docs/manual/usage/upgrading.md

@@ -0,0 +1,148 @@
+# Upgrading to a new Home Manager release {#sec-upgrade-release}
+
+## Overview {#sec-upgrade-release-overview}
+
+When upgrading NixOS to a new major version (e.g., from 24.11 to
+25.05), you also need to upgrade your Home Manager channel to maintain
+compatibility. This guide covers the proper steps to upgrade Home
+Manager channels for NixOS 25.05.
+
+## Understanding Home Manager Versioning {#sec-upgrade-release-understanding-versioning}
+
+Home Manager follows NixOS release cycles and provides corresponding branches:
+
+- **release-##.##**: Stable branch for NixOS ##.## (current stable)
+
+- **master**: Development branch (tracks nixos-unstable)
+
+:::{.note}
+Always use the Home Manager version that matches your NixOS version to
+avoid compatibility issues.
+:::
+
+## Channel-Based Installation (Traditional) {#sec-upgrade-release-understanding-channel}
+
+1. First, verify your current Home Manager channel:
+
+   ``` shell
+   $ nix-channel --list
+   ```
+
+   You should see something like:
+
+   ```
+   home-manager https://github.com/nix-community/home-manager/archive/release-24.11.tar.gz
+   ```
+
+1. Update the Home Manager channel to a NixOS 25.05 compatible version:
+
+   ``` shell
+   $ nix-channel --add https://github.com/nix-community/home-manager/archive/release-25.05.tar.gz home-manager
+   $ nix-channel --update
+   ```
+
+1. Apply the changes:
+
+   ``` shell
+   $ home-manager switch
+   ```
+
+## Flake-Based Installation (Modern) {#sec-upgrade-release-understanding-flake}
+
+If you're using Home Manager with Nix flakes, update your `flake.nix`:
+
+```nix
+{
+  description = "Home Manager configuration";
+
+  inputs = {
+    # Increment release branch for NixOS
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+    home-manager = {
+      # Follow corresponding `release` branch from Home Manager
+      url = "github:nix-community/home-manager/release-25.05";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, home-manager, ... }: {
+    homeConfigurations."yourusername" = home-manager.lib.homeManagerConfiguration {
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+      modules = [ ./home.nix ];
+    };
+  };
+}
+```
+
+Then update and rebuild. If you are using Home Manager standalone:
+
+``` shell
+$ nix flake update
+$ home-manager switch --flake .
+```
+
+And if you are using Home Manager as a NixOS module then you will need
+to update your system configuration instead and run
+
+``` shell
+$ nix flake update
+$ sudo nixos-rebuild switch
+```
+
+## State Version Management {#sec-upgrade-release-state-version}
+
+:::{.warning}
+Careful updating your `home.stateVersion` when upgrading Home Manager.
+:::
+
+The `stateVersion` is best to remain set to the NixOS version you
+**first installed** Home Manager
+
+```nix
+{
+  home.stateVersion = "24.11";  # Example: if you first installed on 24.11
+}
+```
+
+**Why?** The `stateVersion` ensures backward compatibility and
+prevents breaking changes from affecting your existing configuration.
+
+**Remember:** Channel version is not the same as State version. Update
+the channel, keep the `stateVersion` unchanged. Advanced users can
+view the changes between releases and see if any of the `stateVersion`
+changes will affect them and increment, if they migrate their
+configurations to follow the changed evaluation.
+
+## Troubleshooting {#sec-upgrade-release-state-troubleshooting}
+
+### Common Issues {#sec-upgrade-release-state-troubleshooting-common-issues}
+
+Check the [Home Manager Release Notes](#ch-release-notes) for breaking changes.
+
+1. **Version Mismatch Warning**: If you see warnings about version
+   mismatches, ensure your Home Manager version matches your NixOS
+   version.
+
+1. **Module Changes**: Modules are constantly being updated with new
+   features to keep up with changes in upstream packaging or to fix
+   bugs and add features. If you have an unexpected change, check if
+   there was something noted in the release notes or news entries.
+
+1. **Channel Not Found**: If `nix-channel --list` shows no channels,
+   you might be using a different installation method (like flakes or
+   NixOS module).
+
+### Verification {#sec-upgrade-release-state-troubleshooting-verification}
+
+After upgrading, verify the installation:
+
+``` shell
+$ home-manager --version
+```
+
+This should show version 25.05 or indicate it's using the release-25.05 branch.
+
+## Additional Resources {#sec-upgrade-release-resources}
+
+- [NixOS Stable Release Notes](https://nixos.org/manual/nixos/stable/release-notes)
+- [Home Manager Release Notes](#ch-release-notes)