As I recall if you login through full browser, click your user name and there should be a dedicated premium item in the menu. On there is where the cancel option was for me. Which is now gone because I've already cancelled. They just seem to want to give it to me for free.
I have a very weird other problem. I cancelled mine at the start of June after the API information came about. My last day was June 16th and I have a cancellation message to attest to that.
Except it now says July 16th is my last day and they didn't take the money and on the premium option it offers the subscription to me. So I am unsubscribed but, have free premium I don't even want.