– Archive the old HTML site files, you will probably want to stripmine them for content. Delete the files from the webserver.
– Copy a fresh set of WordPress files into the same directory.
– Create a new database and database user + password, configure these into the wp-config.php of the new site.
– Run the install script.
The details are in the install instructions:
https://codex.www.ads-software.com/Installing_WordPress