Go to Settings > Subscription Info, then press 'Click Here'.
Then, the following page will appear, change your subscription plan, then click 'Proceed'.
Finally, confirm changes and required payment.
If you're downgrading your subscription, your new subscription plan will take effect after your current billing period ends. If you're upgrading your subscription, your new subscription plan will take effect immediately, and you'll be issued a prorated charge for the rest of your current billing cycle.