You can unsubscribe/stop any renewals, using the methods CSound1 links but you will also have to deal with the issue you still have to pay for the current subscription.
If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - https://support.apple.com/HT203005 - additional information: https://discussions.apple.com/thread/7824884?answerId=31245423022#31245423022
Payment methods that you can use in the iTunes Store, App Store, and iBooks Store - https://support.apple.com/HT202631
You can request a refund from Apple if you wish. Realize 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.
Get help with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - https://support.apple.com/HT204084
Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"
Other ways to contact iTunes Store Support: