cancel in app purchase from the app without going out to setting ?

You cannot cancel the subscription within the app. You can direct the user to a screen that allows them to manage the subscription outside of the app.

There is no call back when a subscription is 'canceled' by the user. If by 'canceled' you mean 'not scheduled for an automatic renewal' then your app detects this at the end of the current cycle when it checks to see if the app has been renewed. If you mean 'the user called the App Store and demanded a refund' - then there will be nothing until the app choses to referesh the receipt and then it can detect a 'cancellation_date' field - but such cancellations are very very rare.

All your app can do is to open Settings, where the user will then take steps to cancel.

If they're asking how, give them this link;

Learn how to cancel the automatic renewal of your subscriptions.