Secure plan subscriptions are managed through the App and Play store, so they follow their terms and conditions.
If you cancel a subscription, you will have access to subscription features until the end of your billing cycle. You will not receive a refund for any unused time.
if you're upgrading your plan, the change will take place immediately. You will be charged the difference.
In other cases, the change will happen at the beginning of the next billing cycle. You will not receive a refund for any unused time.