In order to inform USCIS about something like this, people would need to inform to the office wherein the case is being processing. But since USCIS is a most bureaucratic agency of US govt. then it is better to write to all of its offices to make sure they know what you wanted to tell them. That means, one can send the copy of the same letter/request to local office, service center, headquarter in Washington D.C., Chicago office, Missouri office. You have nothing to loose by sending your request to all these offices; rather it is to get surety of your request.
But make sure to write full information-like beneficiary's name, date of birth, place of birth, Alien number, Petitioner's name. Also, try to attach any correspondence received from USCIS like receipt notice or something like that.
Good luck.