This depends on which platform you purchased your subscription. Please see below for an explanation of the functionality of the three stores:
On the website During the trial, you are able to upgrade or downgrade to any plan (Silver, Gold, Business) and billing frequency (Annual, Monthly) that you wish. However, once the trial has ended and you have started your subscription, you can only upgrade from monthly to annual if you purchased via the website. If you wish to change your subscription from annual to monthly, we recommend that you cancel the one you are on (you will be able to use the remaining time on your subscription) and change to the new subscription once this expires.
iTunes and Google Play
If you purchased via iTunes or Google Play, you are able to change your plan and any pro rata adjustment will be automatically handled by the relevant store.