Moving from the monthly payment subscription to the Annual one is designed to save you some dollars given the confidence you have placed in the product.
On the web app (see app.invintorywines.com) choose the Prestige tab from the left sided menu and then Subscriptions. You should be able to change the plan from there.
On the mobile app, you can change your subscription plan by clicking on the link below from Apple Support and following the steps.
Learn how to cancel subscriptions that you purchased with an app from the App Store. You can also cancel Apple TV+, Apple News+, Apple Fitness+, and other subscriptions from Apple.