Let’s say I became involved in a new activity after I had applied to Harvard.
And I think it “might increase” (might…might…) my chance of being accepted.
Should I upload it through the portal? If yes, what should I upload? Like a letter explaining my new activity?
Or should I give them an e-mail and say I’ve been involved in it?
- Would it be okay for me to start the letter with "Dear Sir or Madam"?
yes, that’s fine but I think you’re mistaken to imagine that ANY activity that you’ve been involved in for ~2 months will move you any further. I think it’s seriously straw grasping.
Hello, @T26E4! Thank you so much for your reply. Yeah, I don’t even know if it’d find its way to my application file… but maybe it’s better than not sending it in at all… 
Some people have advised faxing in new information instead of uploading it to the portal or sending an e-mail. The theory is that a fax will be most likely to be processed quickly; the portal information may not be looked at again, and an e-mail may not be read for a while. I don’t know how accurate all this is, but if I had to choose one method, I’d pick faxing. (I wouldn’t choose multiple methods—that could annoy the recipients.)
I think part of @T26E4’s point is, never let 'em see you sweat. Sending in the update as you describe it is letting them see you sweat.
It would be more likely to harm your application (although not by much, probably) than to help it.
@Planner Oh nooo… I just submitted it through the portal. I guess I’ll have to stick with the portal… Thank you so much for your reply, though! 
@notjoe Yeah, I agree with you on the “never let them sweat” strategy. Thank you very much for your comment!