To edit a Case Background, head over to the Case Card, Details-view, under the Background section.
Click the Background field and enter the information you want.
After editing the Case Background, save the change by clicking anywhere in an empty space.
Note:
1. Ensure that you are either a Case Manager, a Client Manager, a User with 'Assistant'-role, and a User with 'Administrator'-rights set up to be able to make changes on Case Card.