You may cancel a subscription during its free trial period via the subscription setting in your iTunes account.
Your account will be charged for renewal within 24 hours prior to the end of the current period and will identify the cost of the renewal.
Any unused portion of a free trial period, if offered, will be forfeited when the user purchases a subscription.
This must be done 24 hours before the end of the subscription period to avoid being charged.
Subscription automatically renews unless canceled at least 24 hours before the end of the current period.