Cancelling an Apple subscription
Yes, you can unsubscribe from a PC:

Make sure iTunes is installed, then open it up. From the menu Account choose View Account and sign in with your Apple-ID.

Now select View Account again, and in the Settings section click “Manage” besides Subscriptions.

From the list of subscriptions, press Edit for the HBO subscription you want to cancel. Now select that you want to Cancel, and confirm your cancellation.

An alternative option is to call Apple and let them help you cancel the subscription.