This program makes it as simple as a double click (and using UAC one time) to get back to the legacy version of Microsft Edge.
This tool even works in 20H2 Update!
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The app sets a registry key as described here that forces Windows to launch Edge Legacy instead of "The new Edge":
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\EdgeUpdate
Allowsxs = 1
If you have the new version already installed you have to launch the new edge installer. The new release (v2) now handels this process automatically wile showing a nice waiting dialog.
Have a look at the following section for more details.

According to this post Legacy Edge Launcher now also deletes this registry key: SOFTWARE\WOW6432Node\Microsoft\EdgeUpdate\ClientState\{56EB18F8-B008-4CBD-B6D2-8C97FE7E9062} -> BrowserReplacement

Then the app uses the IApplicationActivationManager COM Interface (as describled here) to start Edge Legacy with the following package name:
Microsoft.MicrosoftEdge_8wekyb3d8bbwe!MicrosoftEdge

You can find the COM-Refrences in ShObjIdl_core.h(eader) and the app logic in ApplicationActivationManager.vb.

Have fun!

Explanation Video

No worries! The method below will still work, but after you have launched the app the first time you have to go to "Settings" -> "Apps" and there search for "Edge".
If you click "Microsoft Edge" you should see a "Modify" Button that will start the Installer of the new Edge. If you reuse the app, the old version of Microsoft Edge should pop up.