I did not succeed in triggering an update email in case an event has been changed.
I filled the template, but do not even know who will be notified. (I hope the registered users and maybe the administrator)
Any idea what to configure for triggering this email?
No, it's not poossible to send event update mails to registrants. There could be many registrants for an event and right now, there is not a reliable way (in Joomla) to deliver emails to many registrants (because most hosting providers limit number of emails can be sent per hour)
So right now, it's not supported but I can try to support it in the future