How to rollback firmware from V 5.0 back to V 4.20

If flow backup isn’t working then you might try community backup option, and if that isn’t working and there is no backup available for 4.2 of your Homey under Homey Backups (you can disable auto backup in the app settings).
Then there is nothing else then restart over from factory defaults if you really want v4.2 over v5.