Within the events module, you have a range of opportunities to communicate with your registrants about your event. This includes on registration, cancelling or reminding or rescheduling their registration.
You can select Edit event type emails from the three quick action dot menu within the event to update the content of these emails. There's a short summary with each email explaining its purpose and when it'll be available to use within the system.
Select the relevant email from the list and then select the Edit email button in the top right of the screen to make your changes.
By default all event types have 24 hour reminder email set. This will be sent to all registrants 24hours before the event start date and time. You can remove this reminder email from your event type in this same area.