For an Update Letter format, I suggest something like this:
Should not exceed one page; the shorter, the better. Address it to the Admissions Committee. Include AAMC ID #.
Say you want them to be aware of updates pertinent to your application in case it may help them make a decision on your file:
a. You might give your fall/spring GPA depending on the time of year, perhaps mentioning some classes relevant to med school.
b. Tell them any newly begun activities and any ongoing activity since you submitted (only the important stuff).
c. A publication is very important; if it's too long, use et al to shorten the author list.
You might close with why their school is perfect for you, trying to mesh with the school's mission statement. Say you look forward to hearing from them in the future regarding interview or acceptance as appropriate.
Then sign your name.