A letter of intent tells one specific school that it's your clear first choice and that you will attend if accepted. Send this to only one school, and only if that's genuinely true — schools do occasionally compare notes, and claiming this to multiple programs can backfire.
An update letter shares meaningful new developments: a new role, a publication, additional clinical hours, or a notably strong recent grade. Keep both types of letters to one page, specific rather than vague, and genuine in tone. Tie any update explicitly back to why it makes you a better fit for that particular school, not just a stronger applicant in general.