You can either
Follow these steps to view, change or cancel subscriptions from your iPhone or iPad:
On iOS, go to Settings.
Tap your Apple ID at the top of the screen.
Tap “Subscriptions” - “Grid Diary”
If you're being charged for a subscription, but you don't see your subscription, make sure you are signed in to the correct Apple ID for Grid Diary’s subscription.
On this tab, you can choose other subscription offerings, or tap “Cancel Subscription” to cancel your subscription. If you choose to cancel, your subscription will stop at the end of the current billing cycle.
About the billing and automatic subscription:
After confirming purchase or completing the free trial, you will be charged from your iTunes account.
The subscription will automatically renew unless you choose to cancel it more than 24 hours before the renewal date.
You will be charged in the 24 hours before your renewal date.
Automatic subscription fee is the same as the initial one.
Purchase confirmation will be received by your iTunes account.