If you subscribed to MangaTime Plus through the Apple App Store, you can manage or cancel it directly from your Apple ID settings.
Follow these steps:
Open the Settings app on your iPhone or iPad.
Tap your name at the top.
Select Subscriptions.
Find MangaTime Plus in the list.
Tap Cancel Subscription and confirm.
Your subscription will stay active until the end of the current billing period.
For more information, see Apple’s official guide: View, change, or cancel your subscriptions