Editing the default plan or creating a new plan for a service is described in Creating and editing service plans.
However, if you have several plans for a single service, you might want new users added to the service due to tracked usage to automatically enroll in a specific plan. The default plan is, per default, the first plan created when you added your service.
If you've added more plans, and want another to be the default:
Navigate to the service
Select Manage Service, then Settings
Select Default Plan
This option is only available if you have more than one plan
If financial data or a usage from a user lacks a designated destination, it will now be placed into your selected default plan.