Reinstall Drupal8

1. Empty the files directory.
2. Replace the settings.php file with a copy of the default.settings.php file.
3. Add write permissions to the settings.php file.
4. Drop all tables in the database.
5. Reinstall the site on http://yoursite.com/core/install.php.

Ref http://dribbit.eu/drupal-8/reinstall-drupal-8