This applies to subscriptions started through our website. If you subscribed through iOS or Android, you can do this through the subscription managers in their App Stores.
To change your billing frequency on the website, open the left sidebar menu and scroll down to the "Account" section and click "Subscription." On this page, you can click "Change Subscription Plan" to change the plan you want to have:

Note: Annual subscriptions are paid up front ($59.99 USD) for savings of $4 a month compared to monthly billing.
If you have any issues or questions about billing, please contact us at support@eatthismuch.com and we'll make things right.