How can I go about cancelling an App Store subscription?

I I subscribed to an apple photo app that allowed me to take out individuals that were in my picture.

It was suppose to be free for a month however but did not cancel the subscription before the month was up. I have been charged now for two months and I not longer need this app. Therefore, I would like to cancel this but I seem to be having a problem doing this. Would appreciate it very much if you would assist me on doing this. The app was Apple Photo and the reference number on my credit card is as follows:

**************. Thanks so much for your help Carol M*******

[Edited by Moderator]