To force a new installation of Matomo (Piwik), you can simply delete the configuration file in config/config.ini.php. When you visit Matomo, it will trigger the Matomo installation process. If you install Matomo specifying an existing database containing Matomo data, the installation process will automatically detect that the Matomo tables already exist. You will be able to choose to either keep your existing data, or delete the tables and start from a clean database.