To return to the configuration with a single Apache web server, stop the Reverse Proxy Server (nginx) service in Tools & Settings > Services Management.
To make nginx the frontend web server again, start the Reverse Proxy Server (nginx) service.
Note: The start and stop operations for the Reverse Proxy Server (nginx) service do not only start and stop nginx, they actually switch the web server configuration (nginx and Apache combination or just Apache as a frontend web server). The restart operation works in the same way as for all other services: the nginx service is restarted.