I was recently surprised to learn the Google app repository browser doesn't let you do that. If you use apps that are also on f-droid, it might be better to install from there to avoid being locked into a new version that you don't like. Or make backups so you have the old apk and can restore the data directory as well (iirc it doesn't let you keep that upon downgrading).

For system images, it's a bit harder. I don't really have a good solution that would downgrade the OS without also restoring an old app state.