You can easily change your subscription plan through your iPhone or iPad settings. Follow these steps:
Open the Settings app on your device.
Tap your name at the top.
Select Subscriptions. If you don’t see Subscriptions, select iTunes & App Store → tap your Apple ID → choose View Apple ID and sign in → scroll down and tap Subscriptions.
Find and tap Fasting app subscription.
Press the See All Plans button.
Select the plan that best suits your needs.
Your subscription will update immediately according to the new plan.