If you subscribed to MangaTime Plus through the Google Play Store, you can manage or cancel it directly from your Google account.
Follow these steps:
Open the Google Play Store on your Android device.
Tap your profile icon (top right).
Select Payments & subscriptions → Subscriptions.
Find MangaTime Plus in the list.
Tap Cancel subscription and confirm your choice.
That’s it, your subscription will remain active until the end of the current billing period.
If you need more details, Google provides an official guide here: Manage or cancel a subscription on Google Play