Unfortunately there’s no easy way of doing this, unless through some specific plugins. However it depends entirely on the context of your website.
One Click Demo Import plugin doesn’t get along well with sites that have existing content so it’s best to start with a clean WP instance.
Using WP Reset plugin
This plugin will reset the entire database of your site however it will keep files intact. This plugin should only be used if you haven’t yet added any custom content, or you have existing sensitive data.
Manually deleting content added by importer
In almost all Rey’s demos, this is the content that is imported and can be manually deleted:
- Pages (eg: https://d.pr/i/nFfDgc )
- Posts Categories
- Products ( eg: https://d.pr/i/U9JTAo )
- Product Categories
- Product Tags
- Product Attributes
- Global Sections ( eg: https://d.pr/i/qSAzSB )
Another issue is resetting the Customizer settings. Try installing Customizer Reset plugin and reset it (and remove the plugin afterwards).
After doing this you should be able to import a new demo properly.