Hi,
Thanks for reaching out!
If you want the same demo content then you need to run the demo importer again it will overwrite all the data. But if you want to delete existing data then you can reset your wordpress. Install the WP Reset plugin from Plugins -> Add New, after successful installation activate the plugin and reset the site. There are many options available for resetting, read it carefully and then proceed, as it will completely clean your WordPress. After reset install the theme again and run the demo importer.
If you have any questions feel free to contact us.
Thanks