I'd recommend checking what the school suggests. Some schools prefer no contact, while others encourage updates. If you can't find the info online, I would think a (short) polite email or phone call would work.
This is a pretty helpful article on steps you can take:
https://www.ivyleagueadmission.com/waitbus.htmlSome people will provide extra recommendations, retake the GMAT, etc. I think the important thing is being able to identify the weakness in your application and demonstrate that you're working on improving it. It seems to me, also, that applicants often have strong enough numbers, but are weaker in possibly the most important part of the application - the elusive answer to the Why MBA, Why XYZ School question. If you can really pinpoint that and demonstrate your genuine interest in Wisconsin's program (what do you plan to do, which of Wisconsin's strengths appeal to you, and how you are going to make that school famous and give back tons of money one day). Reiterate your interest with a short letter (no more than a page) listing your areas of improvement, and give the AdCom the feeling that you really have a concrete sense of yourself and a plan for your future that necessitates an MBA.
Good luck!!