To upgrade or downgrade your subscription, go to Account Settings > Billing and click on Subscription Details.
Next, you'll be redirected to your Billing information section. Click the Choose your plan button to check the available plans and select the one that best suits your needs 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.