You must first cancel any subscription automatic renewal (if you simply delete an app this does not cancel subscriptions):
Manage your Apple Music subscription - Apple Support -"Learn how to change from an Individual membership to a Family membership, turn off automatic renewal, or join Apple Music later." Turn off automatic renewal and your membership will finish at the end of the current subscription period. --> https://support.apple.com/HT204939
If you wish a refund: Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no.
For a subscription refund you will have to make a special request using one of the contact links below.
https://support.apple.com/HT204084
Contact iTunes Support:
https://www.apple.com/emea/support/itunes/contact.html
https://support.apple.com/country-selector/itunes