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.
On iOS tap device Settings > iTunes & App Store > Apple ID > view Apple ID > Subscriptions > Verv and choose the appropriate option from the resubscribe list. Note that Apple charges right away for the whole year and you receive a refund for the remainder of the monthly period automatically.
On Android first cancel the ongoing account in your Google Play Settings. Once a monthly term has expired use in-app banners to resubscribe. Remember that Google does not divide the amount up and charges one lump sum.