A lot of schools specify on their FAQs. Some clearly state that if you have any updates send email or mail to ___. Generally, in other threads, I think snail mail was preferred, and some people sent an e-mail to alert them of the incoming letter. Check with the school first, though.
i think snail mail is generally what people consider the best option. for all of my updates though i used email. some of the schools that like paper emailed me back asking for pdf copies of stuff, and it all seemed to work out.