Call the admissions office and ask them directly if they accept update letters and if you can upload one to your application portal. Make sure to tell them you were invited to interview, this lets them know that you are asking a logistical question and not a hypothetical. If you choose to email the admin office, include Interview Scheduling Inquiry in the subject line of your email.
If the admissions office says they do not accept letters, ask them how to best communicate the update (say, in the interview directly perhaps?). if they do accept update letters or ask you to defer to your own discretion, then you're free to go ahead and upload directly!