You can change the billing period for your subscription through the subscription management screen on your device:
On iOS, you can manage your app subscriptions through the Settings app on your device.
On Android (Google Play), you can manage your app subscriptions through the Google Play store app. Subscription settings can be found in the 'Account' section of the app.
On Amazon Fire devices, you can manage your app subscriptions by navigating to "Apps" on the device homescreen, selecting "Store" and then selecting "Manage Subscriptions" from the menu.
When the billing period is changed, the new setting comes into effect at the end of the current period. The subscription fee for the whole of the next period (1 month or 12 months) will be taken at the start of the next billing period and will then recur at the same interval unless Automatic Renewal is disabled.