You may cancel the free trial at any time through iTunes.
See this link for a step by step guide: https://support.apple.com/en-us/HT202039.
If you cancel your free trial, you will not be charged at the end of the 7-day free trial period.
Disabling the automatic renewal must be done 24-hours before the end of the subscription period to avoid being charged. Due to Apple’s policy, immediate cancellation of subscriptions isn’t possible.
If you run into any other issues related to billing, please contact iTunes support at https://getsupport.apple.com/.