Any Garmin dealer can do it. I believe it requires a Windows program and serial connections to the back of the 327 along with power/ground. I don't recall exactly, but takes about 15 minutes or so. The files are available on the Dealer side of the Garmin website.
I would only bother with the upgrade if you have a specific need (like ADS-B support). My 327 was so old it didn't even have the RS232 setup page in the menu.