You can cancel your subscription in the iTunes trial settings at least 24 hours before the end of the free trial period.
* Please note: any part not used during the free trial period (if offered) will be forfeited upon purchasing a premium subscription during the free trial period.
* You can cancel a free trial or subscription at any time by canceling your subscription in your iTunes account settings.
This should be done 24 hours before the end of the free trial or subscription period to avoid being charged.
* With a free trial period, the subscription will automatically renew to the paid subscription.