Please find below the steps 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. select the coupon to be redeemed by clicking on Redeem Coupon next to the name of the coupon
4. before paying leave everything as it is (with the boxes selected) and click on Profile
5. click on Coupons and then click on Info to 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.
6. Go back to the payment window and complete the payment by clicking on Pay Now.