You can change your billing period from annual to monthly on the pricing page. Basically, all the necessary info is already there.
If you decide to stay on the same level you’re currently on, your monthly subscription will start only after the annual plan ends.
If you decide to upgrade, you won’t be charged immediately: the unused period of your current subscription will be converted in accordance with our price ratio, so the duration of your subscription will be shortened. Once this period ends, the first payment for a monthly plan will be taken, and your subscription will be extended on a monthly basis since that moment.
Finally, if you choose to downgrade, the monthly subscription of a lower level will go live only once your current annual plan ends. Again, the first payment for the monthly plan will be made only once your ongoing annual subscription expires.