You may upgrade or downgrade freely at anytime, the only exception being when your account is too big for a smaller plan (e.g. if you have 6 active clients, the 5-client plan is not available).
Click on your headshot in the upper right >> My Account >> Subscription to view and change your subscription plan. Simply click on your preferred plan to upgrade or downgrade your subscription.
When you make any changes to your subscription plan, the credit card that you provided will be charged the new rate on your next billing cycle.