Open Subscription & Help and choose Manage. On Mac it's in the Dayla menu, on iPhone in the Review's menu. Apple handles the subscription, so refunds go through reportaproblem.apple.com.
Open Subscription & Help and choose Restore Purchase, with the same Apple Account.
Export All Days gives you everything in one file, with or without a subscription. It's in the File menu on Mac, and in the Review's menu on iPhone.