It is not possible to change domain PHP handler type under the Hosting Settings. Handler is set to "Run PHP as FastCGI Process Manager" and other PHP handlers are not available for selection.
The "Process PHP by nginx" option is enabled under the Web Server Settings for the domain in question. With this option enabled PHP files (.php) will be handled by nginx and PHP-FPM and so the FastCGI is used.
Go to Websites and Domains > Web Server Settings > disable "Process PHP by nginx"