If you want to fresh start your web site for demo importing.
First, try installing “wordpress reset” plugin, go to “Tools > Reset” section, type the reset in input box.
https://wordpress.org/plugins/wordpress-reset/
Then accessing to your wordpress installation via ftp or hosting control panel to remove all contents in wp-content/upload folder
After this, all the data will be new ! :). Activate your theme, plugins and start creating your site.