Looks like you're thinking of getting a yearly subscription 🙂 And that's great - you can get the most of Premium for the whole year at a significantly discounted price.
To do that, find your subscription: device Settings > iTunes & App Store > Apple ID > view Apple ID > Subscriptions > choose the app.
Then simply switch to the Annual Premium from the list under your active subscription.
You will be charged right away for the whole year and automatically receive a refund for the remainder of the monthly period.