If one of your client's customers mistakenly entered the incorrect amount in the coupon's Subtotal Field during the redemption, here's how you can fix it:

First, just go to your Pages list and find the Coupon (Coupon Page) that was redeemed. Then click the number of redemptions that is under the Redemption column.

The Redemptions Page will open and will show all the redemptions done on the coupon.

From here, the coupon value can be edited.

Click on update and that's it.