When I tried changing the channel to verify bug 1801201, where we setup a temporary nightlytest rule for the Widevine CDM update, I found changing app.update.channel in about:config had no effect. I had to do a local build with a modified pref in:
https://searchfox.org/mozilla-central/rev/aa329cf7506ddd966542e642ec00223fd7461599/browser/app/profile/channel-prefs.js#9

It appears others have the issue, as documented here:
https://searchfox.org/mozilla-central/rev/aa329cf7506ddd966542e642ec00223fd7461599/browser/components/newtab/docs/v2-system-addon/geo_locale.md#7

And previously filed under bug 598594. In that bug we have some discussion, and we used to be able to work around by changing another pref file in the profile directory, but that file appears to no longer exist.

It would be nice if we could override the value for testing Widevine updates, even if just on nightly builds.

app.update.channel is set in channel-prefs.js in the install directory, not in the profile.

As far as I know, this is working as intended. Exposing the pref in about:config also makes it susceptible to hijacking.

The above two comments are correct. This is working as intended. You are not meant to be able to change channels by changing app.update.channel. I believe that there are good reasons for this. One is the reason that :RyanVM gave. Another is that different profiles can have conflicting prefs. We don't really want Firefox to switch back and forth between channels every time that a different user logs in.