I applied RD to a handful of schools and today I just placed in a regional science fair with a continuation of a research project that I mentioned in my app. Should I update the schools about it and if so how?
If your application shows that you’re interested in science (i.e. majoring in chemistry, have completed research, etc), then yes update them.
A quick email to your regional admissions officer. “I wanted to let you know that I recently placed 2nd in the Boston Science Organization and qualified for the National Competition.”
^^^Agree. It is fine to send a brief update. This is not really a new EC, it sounds like a continuation of something that was on your application already.