To force a new installation of Piwik, you can simply delete the configuration file in config/config.ini.php. When you visit Piwik, it will trigger the Piwik installation process. If you install Piwik specifying an existing database containing Piwik data, the installation process will automatically detect that the Piwik 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.