The easiest is uninstall and then install the version you want from the list at the bottom. If the version you want is so old that it doesnt show up in the list anymore, then you have to go to the git repository to get it.

Not all apps support downgrading btw, so if you just try to install the old version over the new one without removing that first, the app might not function properly.

IMHO you need to use adb to force install older version. Something like :
adb install -r -d myapp.apk

Look it up! It should be possible to use termux on phone and connect wirelessly to your own phone to access adb. Or you have to use another device to connect to your phone.

Of course simpler solution is being able to backup settings and data, uninstall then install older version and restore data.

Usually I click on source and go to the git repo and download the apk. Also take care to not update the app automatically until a new release is out.

If you go to the repository settings there's an option to enable archived apps and outdated versions. Then select the f-droid archive repo for that app.

Often these are disabled for a reason, so you might want to check if there's any important bug fixes since then.

@transscribe7891 Archived versions are kept, yes, see https://f-droid.org/2024/05/02/twif.html#f-droid-core