You can do that from your settings page - https://seekingalpha.com/account/subscriptions_settings. Just look for the name of your service and click on the upgrade to annual button, and you will be charged for the annual rate as of the next billing date.
If you don't see that button, it means your monthly rate is the same or lower than the current annual rate, so there are no savings available.
Please note that the annual payment is not refundable once billed in an upgrade. You can contact us at email@example.com if you change your mind before the billing.
Customer support service by UserEcho