Hi there,
As others have answered this question, I will add what has not been mentioned. If you expect that you will have more things to update upon, it may be a good idea to wait so that you can include it all in one letter, rather than sending too many letters. You can of course include it in your thank you letter, which should be send out 1-3 days after your interview.
And for your top school of choice, you can send a letter of intent/interest and can include this update within that letter. This, however, should only be sent to one school. Essentially this letter will state that if you are accepted to this school you will accept the invitation as they are your top choice. We usually suggest sending a letter of intent around one month after your interview. This letter can be helpful for the committee in deciding who to send out those last acceptances to, and therefore is a great idea if you are sure you will accept the invite. You can add your update within that letter (after you thank them for the interview and reiterate your interest and commitment).
Best of luck!