To upgrade your trial account to a paid account, click the Upgrade-link in the yellow box in the top navigation, or go to Organization > Account > Subscription.
This brings you to the subscription management page detailing your existing subscription, and providing you with the option of upgrading to a paid account.
To upgrade to a paid account, click the "Switch to this plan..." button in relation to the plan you would like to upgrade to. Once your existing plan expires, you will be switched over to the new plan.
To change the email address of the invoice recipient, click the pencil-icon in relation to the "Billing details"-section.
