How to notify admissions that one quit another program?
The important thing here is that you realize that you do need to report this information to schools to which you will be applying, as it will matter in their deliberations over your application. That said, you do have some degree of latitude in finding the best way to inform the schools of this. If they have a "special notes" or "additional information" question in the application, that might be a good place to put such information. Otherwise, you would want to mention it somewhere in the cover letter (if you get to include one) or personal statement (if you don't). 

However, this information should be provided by you; don't leave it for the people writing your recommendation letters to mention. That will just raise more red flags.