You can change existing banding scenarios in 2 different ways.
1. Clicking the EDIT button in the banding table associated to the banding scenario will open a pop-up window similiar to the initial window used to create the banding scenario where any changes can be made (refer to the proper support article for more details on creating banding scenarios).
2. After entering the banding scenario (by clicking the DETAIL button in the Banding table) you can make directly in the details page or by clicking the edit banding scenario button (depending on the type of banding scenario created).