How do you cancel a monthly change on an app

If it isn't a subscription, the other possibility for a monthly charge is for an upgraded iCloud storage plan (i.e., large that the free 5GB plan). Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in the following Apple article: Downgrade or cancel your iCloud+ plan - Apple Support

If it isn't that either, then use the following instructions to check for other possibilities (and to contact Apple Support if that doesn't help either)>

This is primarily a user-to-user support forum; other Apple users like you. We don't have access to information about charges to your account. Apple has a support article about charges — see each of the steps in the following instructions. They include information on contacting Apple Support if the other steps don't help. See: If you see 'apple.com/bill' on your billing statement - Apple Support

There is an Apple article that explains how to cancel subscriptions. Click on the following link: If you want to cancel a subscription from Apple - Apple Support