I really have the highest opinion for @Goro's and @Mr.Smile12's advice and contribution to this forum, but in this case I respectfully disagree. Some schools welcome updates and if some of your schools welcome updates, I would send them an update about the award and your role in earning it, especially if you hadn't written much about this in your applications. If you had written about this work and the aspects of it that earned the award, I'd keep the update a lot shorter or maybe skip it.
Everything you do and every interaction with a school demonstrates your judgement and maturity. So the answer to your question really depends on the content of the letter, the school you are sending the letter to, and what's in the rest of your application to that school. I can easily imagine letters that would have the potential to damage; for example, they are long-winded and duplicative of other parts of the application. IOW a waste of time. A succinct, update letter that adds information and insight sent to a school that welcomes updates seems to be a low-risk endeavor that not only provides more reasons to accept but demonstrates continuing interest in that program, which welcomes that contact.
