There is no way to edit a month that has passed. In order to adjust those fees, we need to adjust future or current months.
From the family's main page:
1. Hover over the blue box with the white chevron on the left hand side. 2. Select Edit Billing Details. 3. Adjust the current, or future month for the fee by entering in a negative amount. 4. Enter in a reason 5. Click Save Changes