Changing from monthly to yearly subscription

This would depend on where you initially subscribed.

Web
Subscriptions purchased on the myfitnesspal.com website are managed on the website.
Log in to http://www.myfitnesspal.com and click My Home then Premium
On the Premium detail page, click Subscription Settings in the upper right corner.
Set the Auto-Renewal menu from "On" to "Off" to opt out of Premium at the end of your current billing cycle.

Android
-Open the Play Store App
-Select the Menu
-Select Subscriptions
-Select MyFitnessPal Subscription
-Cancel Subscription at the bottom

iOS
You will need to first cancel your monthly subscription. Once you have done that and the subscription has expired, you should be able to create a new subscription for annual.

To do this you will want to return to your MyFitnessPal app. Open the More... menu in the lower right corner. Select Settings, then Subscription Status and click on Manage Subscription.