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

I had to do this recently, an upgrade resulted in an app crashing a few seconds after it was launched. Fortunately the app involved didn’t have a lot of settings so I backed up the data, uninstalled the app then reinstalled the older version from F-Droid (a new install allows one to choose any version available on F-Droid). App now works so I’m happy but I need to contact the dev about the crash.