Terms of service
You are allowed to use the apps according to the standard App Store terms of service.
If you take out a subscription through the App Store then it is between you and Apple. We are not allowed to start, alter, cancel or refund subscriptions. Purchased subscriptions renew themselves automatically until you tell them to stop auto-renewing. Apple’s instructions are here. You can’t cancel a subscription once it has started, but you can tell it not to renew itself in future. You won’t get a refund but you will get the benefit of the subscription until the next renewal date. (Note: be sure to turn off auto-renewal before the last date of the current subscription period, because Apple take payment for the new period up to 24 hours before the old one expires).
Once you have subscribed for a given period, you will get the benefit of the days covered by that period for ever. For example: if you subscribe on the 4th of January 2018 and turn off auto-renewal on the 5th, you will not get a refund; your subscription will run until the 4th of February and then stop; and for ever after, you will be able to look at the 32 days between the 4th of January and the 4th of February 2018, inclusive, and make use of all their subscription content.