Its easy as ABC - on the main menu of your account click "Account Settings" and then go to the "Subscription" tab and click "Upgrade",
choose the subscription duration (monthly or annual). Free PRO trial is available to all Basic and Advanced users who never had a PRO subscription before.
You will need to provide your payment info in order to start a free trial and right after doing this, the PRO plan will become active on your account. Free PRO trial period is available one time per account.
Note: As of now, users can sign up for a free trial only with a credit/debit card (Visa or Mastercard)