Go to the
Navionics web site and fine the map update page - download the update app ( available for PC or Mac )
Plug in your card with a suitable card reader and the app should automatically see it - sign in to you Navionics account ( if you don’t have one you will need to register and activate/associate your card with the account and you need an active subscription to the map regions you want to update.
Once you have done all that updating can take from 30 to 120 min depending on how much needs downloading.