Email [email protected]. A person reads every message, usually within two working days.
Open Settings in the app and tap Restore Purchases, signed in to the same Apple ID you bought with.
Settings → Your account → Delete account. You have 7 days to change your mind, or you can choose immediate erasure. See the privacy policy for what is deleted.
Purchases are made through Apple, so refunds are too: reportaproblem.apple.com.
Where melatonin needs a prescription, including in the UK, the app does not suggest a dose. It is never medical advice; ask a doctor.