While using The Next Ad you can simply manage your subscriptions from the Subscription Manager. In this overview you can see to which apps you're currently subscribed and if it's a free tier or a paid tier.
By clicking the "Manage" button for a specific app you can edit your subscription for that app. You can downgrade and upgrade at any time.
If the app has a usage limit, then you'll see how far the usage is for this month.
You can always downgrade to the free tier to keep your data if you plan on returning later. However, inactive accounts will be cancelled and removed after a period of 6 months. Please keep in mind that data will be permanently deleted after this time period.