Reminders go out automatically to every scheduled interview: roughly 24 hours before and again about an hour ahead. SMS is used when the applicant has a number and has not opted out, otherwise email. Quiet hours apply to the SMS reminder, which is why the day-before reminder goes out in the afternoon rather than at midnight.
Editing reminder wording
Reminders use the templates named Interview reminder (SMS) and Interview reminder (email) in Settings, Templates. Edit them like any other template. {{interview_time}} is rendered in the workspace timezone.
Rescheduling and cancelling
Open the interview from Interviews or the applicant record to change the time, or cancel it. A cancelled interview does not move the applicant back a stage; do that by hand if needed.
No-shows
After the scheduled time, mark the interview Completed or No-show. No-shows appear on the applicant record and in analytics so you can see which sources produce applicants who actually turn up.