Once you decided that you want to upgrade your plan, you can easily do it from the control panel.
1-Click on the "UPGRADE" button in the control panel, on the top right corner of your screen.
2-Select the plan that's best for you.
3-Click "SUBSCRIBE" and fill out your billing information.
4-Refresh your page and your plan will be valid immediately.
Your billing information and invoices are under "Account Settings". To access your account, click on your email adress on the top right.