To change account type, click on the username on top right section of the screen. You will see a drop down. Select Payments from Account Settings.
You will be directed to the billing page. It has 2 sections:
You can change your account type while paying the monthly subscription fees, by changing the option in dropdown labeled Account type. The account type will be valid for 30 days starting from the current expiry date of your account, even if you make an early payment. You have the flexibility to change your account type with every payment & the new account type will be activated immediately after you make the payment.