How do I enable HTTP/2 for site/server on ISPConfig Apache

I am running (Ubuntu 16.04.4 LTS (Xenial Xerus)) ISPConfig 3.1.13 and Apache - I believe enabling and using HTTP/2 would improve page load through parallel downloads.

Can I do this on the server and how?
I tried the following, based on other posts (but it killed Apache and I had to undo):

Code:

root@webserver:~# a2enmod mpm_event
Considering conflict mpm_worker for mpm_event:
Considering conflict mpm_prefork for mpm_event:
ERROR: Module mpm_prefork is enabled - cannot proceed due to conflicts. It needs to be disabled first!
root@webserver:~# a2dismod mpm_prefork
Module mpm_prefork disabled.
To activate the new configuration, you need to run:
  service apache2 restart
root@webserver:~# service apache2 restart
Job for apache2.service failed because the control process exited with error code. See "systemctl status apache2.service" and "journalctl -xe" for details.
root@webserver:~# a2enmod mpm_prefork
Considering conflict mpm_event for mpm_prefork:
Considering conflict mpm_worker for mpm_prefork:
Enabling module mpm_prefork.
To activate the new configuration, you need to run:
  service apache2 restart
root@webserver:~# service apache2 restart
root@webserver:~#

Thanks,