You can upgrade or downgrade your plan from the Manage Subscription page. If you're not already logged in, log in first.
Scroll down past your current subscription details, and you'll see the other plans.
If you're trying to change from a yearly plan to an upgraded monthly plan, the website won't let you make that change immediately, but billing support can. Email [email protected] for help.
Upgrading
When you change to a higher-tier plan, you'll have the option to make the change effective immediately or at the end of your current billing cycle.
If you upgrade immediately:
You'll get a discount on your new plan based on the unused Fast time on your current plan. If you've used all your Fast time, your new plan will cost full price.
Your subscription will reset immediately. You'll get all the features of your new plan including Fast time right away, and your next renewal will be one month (or one year, if you chose a yearly plan) from your upgrade date.
Downgrading
When you change to a lower-tier plan, the change will take effect when your subscription would next renew. Downgrading your plan will not refund your current plan. For information about refund eligibility, see this article.