Any of the multitudes of avionics vendors in the big hangars: A, B, C or D or the Jeppesen tent can update it.
I used to always get my portable GPS updated at one of the avionics vendors in the hangars, in the past there would always be one of them that was $5 cheaper than the rest, and there would be a big long waiting line there. In the past two years, all of them have had uniform pricing... no more extra discount to be found anywhere at Oshkosh, so for more convenience I have just been dropping my GPS off at the Jeppesen tent and pick it up a couple hours later when its ready while I go eat lunch, go shopping at the Fly Market, and wander around the big ABCD hangars.