How to install an old version of an app (downgrade) on F-Droid 2.0-alpha9?

Maybe worth noting: If you manage to install an older version of an app, it still depends on the app and the changes made between versions whether the app can properly run. If an installed update for example changed the underlying data structure for stored data or configuration, the old version might not be able to deal with that. It might error in some way or even crash.