After choosing Upgrade to Verified from your edX dashboard, enter any coupon codes you may have been given for courses on the payment page.
Before you place your order, enter your coupon code in the Add coupon code form and then click the Apply button.
The discount will show up once you click Apply, and the final amount will be decreased.
You can cancel your enrollment for a refund, then re-enroll and make a new payment using the coupon if you forgot to apply your code before clicking Place Order.