Stop your cron daemon. Goto settings, activate php-fpm, check that all other settings are suitable for your system.
Also check that you have "use mod_proxy" enabled in the settings.
Check under "fpm versions" (left side menu) that the executable and paths are correct.
Goto configuration and go through Webserver config-templates, system -> libnss-extrausers and of course php-fpm.
Don't forget to re-enable Cron daemon again.
Webserver Change is also possible but you need to adjust a few paths in the settings according to the Webserver. Also when given through every config you might need to stop the old webserver and restart the new one due to the binding on the same ports.