Did one of your client's customers mistakenly entered the incorrect amount in the coupon's Subtotal Field? No worries, you can 100% edit the amount they entered.
First, just go to your Pages list and find the Coupon (Coupon Page) that was redeemed. Then click its 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.