If you purchased your subscription via the App Store or Google Play Store, here's how to cancel:
Tap your iPhone's Setting's icon > iTunes & App Store > Apple ID > View Apple ID. Enter your Apple ID and scroll down to select the PicMonkey app and cancel your subscription.
Google Play Store:
From Settings, tap Membership and then tap the Google Play Store link at the bottom of the page. Tap Subscriptions from your account page and choose the PicMonkey app from the list to cancel your subscription.
Remember that any changes you make will be reflected across all PicMonkey products. Cancelling your subscription will stop future billing, but will not refund you for charges already put on your card.