[update] Add option --update-to, including to nightly (#6220)

* By default, stable will only update to stable, and nightly to nightly

Authored by: Grub4K, bashonly, pukkandan

Co-authored-by: bashonly <88596187+bashonly@users.noreply.github.com>

README.md

@@ -120,7 +120,9 @@ yt-dlp is a [youtube-dl](https://github.com/ytdl-org/youtube-dl) fork based on t
 
 * **Plugins**: Extractors and PostProcessors can be loaded from an external file. See [plugins](#plugins) for details
 
-* **Self-updater**: The releases can be updated using `yt-dlp -U`
+* **Self updater**: The releases can be updated using `yt-dlp -U`, and downgraded using `--update-to` if required
+
+* **Nightly builds**: [Automated nightly builds](#update-channels) can be used with `--update-to nightly`
 
 See [changelog](Changelog.md) or [commits](https://github.com/yt-dlp/yt-dlp/commits) for the full list of changes
 
@@ -187,6 +189,20 @@ If you [installed with PIP](https://github.com/yt-dlp/yt-dlp/wiki/Installation#w
 
 For other third-party package managers, see [the wiki](https://github.com/yt-dlp/yt-dlp/wiki/Installation#third-party-package-managers) or refer their documentation
 
+<a id="update-channels"/>
+
+There are currently two release channels for binaries, `stable` and `nightly`.
+`stable` releases are what the program will update to by default, and have had many of their changes tested by users of the master branch.
+`nightly` releases are built after each push to the master branch, and will have the most recent fixes and additions, but also have the potential for bugs.
+The latest `nightly` is available as a [pre-release from this repository](https://github.com/yt-dlp/yt-dlp/releases/tag/nightly), and all `nightly` releases are [archived in their own repo](https://github.com/yt-dlp/yt-dlp-nightly-builds/releases).
+
+When using `--update`/`-U`, a release binary will only update to its current channel.
+This release channel can be changed by using the `--update-to` option. `--update-to` can also be used to upgrade or downgrade to specific tags from a channel.
+
+Example usage:
+* `yt-dlp --update-to nightly` change to `nightly` channel and update to its latest release
+* `yt-dlp --update-to stable@2023.02.17` upgrade/downgrade to release to `stable` channel tag `2023.02.17`
+* `yt-dlp --update-to 2023.01.06` upgrade/downgrade to tag `2023.01.06` if it exists on the current channel
 
 <!-- MANPAGE: BEGIN EXCLUDED SECTION -->
 ## RELEASE FILES
@@ -335,6 +351,11 @@ If you fork the project on GitHub, you can run your fork's [build workflow](.git
     --version                       Print program version and exit
     -U, --update                    Update this program to the latest version
     --no-update                     Do not check for updates (default)
+    --update-to [CHANNEL]@[TAG]     Upgrade/downgrade to a specific version.
+                                    CHANNEL and TAG defaults to "stable" and
+                                    "latest" respectively if ommited; See
+                                    "UPDATE" for details. Supported channels:
+                                    stable, nightly
     -i, --ignore-errors             Ignore download and postprocessing errors.
                                     The download will be considered successful
                                     even if the postprocessing fails