To change your subscription on Android, please cancel your current subscription following the steps here. When this subscription expires, the Read Your Body app will prompt you to activate another subscription - at this point you can choose the different monthly / annual option as you please.
To change your subscription on iOS, open the settings app on your device and tap on your name, then tap 'Subscriptions' and tap on the subscription that you want to manage (Read Your Body), then you should see the option to change between monthly and annual.