The Appointment Reminder email is sent to clients usually 24 hours before the appointment start time (although you can adjust that). The text for the default appointment reminder email reads:
Subject line: (Appointment Reminder) Staff Name on Appointment Date & Time
Email Body: Business Name
Your appointment is coming up!
Hey Client First Name,
This is a friendly reminder that your Service Name appointment with Staff Name at Location Name is scheduled for Date Time Client Timezone. If you have questions before your appointment, use the contact details below to get in touch with us.
To cancel or reschedule your appointment before the scheduled time, please click: Manage Appointment URL
Next to the Email to Client line, switch the default 24 hours to whatever other time increment you want:
After you have made the change, press the green "Save Changes" button at the bottom of the screen to commit the update to the hours before the appointment start time that the reminder email is set to go out:
You can choose whether you want to send out another email about this (an appointment edited/rescheduled email template would be delivered to either client and/or staff) or if you'd rather, you can just save without sending any email out to either staff or client.