From within the app you can navigate to your account screen by tapping on the icon in the upper right hand corner of the screen.
If you have subscribed with iTunes and you are on an apple device you will find a button at the bottom of the screen to cancel which will take you to iTunes to cancel. Likewise if you have subscribed with Google and you are on an android device you the cancel button on your account screen will take you to your Google account to cancel.
Alternatively, you can go directly to your iTunes or Google account and go to your App or Play Store purchases and cancel from there. See the links below.
Apple Subscribers: https://support.apple.com/en-ie/HT202039
Google Play Subscribers: https://support.google.com/googleplay/answer/7018481?hl=en