The quickest way is in Nim: Settings > Back up and sync > Delete account. It deletes your account and everything backed up, right away, and removes Nim's data from that phone.
No longer have Nim installed? Email [email protected] from the address you sign in with (or tell us it's a "Hide My Email" address from Apple), with the subject "Delete my account". We'll confirm it's you and delete it within 30 days.
What gets deleted: your email and all backed up forecasts, worries, letters and settings.
What stays: your account ID, with nothing else, for up to 30 days so your other phones know to clear themselves; data on a phone that stays offline for longer than that (use Delete all data in Settings); and anonymous crash reports, which Sentry keeps for up to 90 days.
Deleting your account doesn't cancel a subscription. Cancel it in your App Store or Google Play settings.