How do I update the Subscription Product Bill Line field after creating a credit memo?

You can update the Subscription Product Bill Line field using server scripts in the Application Composer.

Server scripts help you create triggers for custom objects.

To create the trigger you want:

1. Sign in to the application as a setup user.
2. Ensure that you're working in an active sandbox.
3. Navigate to the Application Composer.
4. Navigate to Subscription > Subscription Product > Bill Line object > Server Script.
5. Create the trigger to update the Subscription Product Bill Line field.

To know more about how to user server scripts, see Server Scripts in the Configuring Applications Using Application Composer guide.