Bought on the website: sign in at 1kr.uk and open Billing. You see each subscription, whether it renews or ends, and when. Manage billing opens our reseller's secure page: Invoices, the payment method and cancelling are on our reseller's secure page.
When you cancel, the plan keeps running until the end of the period you paid for ("cancelled, runs to the end of the period"), then the app goes back to the free plan (or to Starter shared from your other app). Your notes, messages and settings stay.
Bought on iPhone (App Store): open the iPhone's Settings → Apple ID → Subscriptions (or Manage subscription in 1krNote's Plans). Apple handles the payment, the invoices and refunds.
Changing plans: on the website, switch through Manage billing; on iPhone, through the App Store subscription page.
Deleting your account doesn't cancel a subscription by itself; cancel first (see "Deleting your account").