To convert your subscription from a monthly to an annual subscription (which comes with a 17% discount), you must first cancel your existing subscription to Sentinel Hub services. Follow the steps described here to cancel your subscription.
Just before the end of the billing period of your cancelled subscription, set up a new subscription with an annual option in the Sentinel Hub Dasboard under the Plans tab.
Follow the same steps if you want to switch from an annual subscription to a monthly subscription.