To upgrade or downgrade your subscription go to Account Settings > Billing. Click on Subscription Details.
After that you'll be redirected to your Billing information section. Go to Your Subscription and click on the button on Select your billing plan if you have a trial account.
If you already have a paid account, click on Change plan to upgrade or downgrade your plan. You will then get a pop-up with all available plans from which you can choose the one that fits you best.