With these tips, there should be a way to cancel a subscription from In-App.
But due to lack of deeper insight into the implementation, I can't make this work.
I can't find the policy right now, but I'm pretty sure there's a requirement from Apple that if you have a subscription in an app you must provide a link to the subscription details in the user's account - so you should have the link that Erel provided in there anyway.
I've had iOS apps rejected at review in the past for not having it.