You can update your subscription at any time from your Account Settings.
Your active subscription will show a "Current plan" and highlighted.
To switch to a different plan, simply click the "Switch to this plan" on the new plan you'd like to upgrade or downgrade to. You can switch to monthly or annual by clicking the toggle above the plans.
When prompted, confirm the change and the subscription will update right away.
If your subscription is active, and you are switching to a monthly plan from annual, the prorated difference will be added as credit towards your monthly subscription.
If you are grandfathered into legacy plans, your current plan will not be visible.