If you have a promo code and want to apply it you need to click "Add promo code" button when upgrading your account. 

Upgrading your account - click "Add promo code" in the Billing page of your account, type your code in and click "Submit"


Observe that the price to be paid for the selected plan changed, and there is a confirmation text on the screen indicating that your promo code is valid

Did this answer your question?