If you want to start fresh, to remove your dummy data and start importing your own, you can do that by going to
Administration > Database/Backup > Clean Database
With using this widget, you will successfully clean everything, so you can start populating the database from scratch.
Delete all transactions
But, if your goal is to delete all the generated documents, your sales and purchasing history and the generated work orders, the option you should use is 'delete all transactions'. That way, your inventory will remain untouched, and your customers and suppliers lists unaffected.
You can do this by going to:
Administration > Database/Backup > Delete all transactions