Steps to follow to change the value of a coupon:
1. Open the user's profile and add a coupon (if not already there)
2. sell an item and click on Redeem Coupon
3. before paying leave everything as it is and go back to the user's profile under the Coupon section. Click on Info and change the value of the coupon. Then save the changes. Please bear in mind that the value of the coupon has to be smaller than the price of the item you are selling.
4. Go back to the payment window and complete the payment.