Current first year student here! Last year there was a lot of waitlist movement around May and June as final seat deposits were due for other schools, and a very steady trickle throughout the summer. I was waitlisted and sent a letter of intent, and a few updates including my final grades and work experiences. I just typed mine up into a word document and sent them as a PDF in an email to the admissions office. You can call the admissions office to ask what they would prefer. I really think sending updates shows your interest and increases your chances of getting off the waitlist!