Garmin is a company that offers a wide variety of electronic devices, among them global positioning satellite (GPS) navigation systems.
If it matches the version currently installed on your Street Pilot c340, you don't need to download a new map.
To get started simply register your new product on the my Garmin website and you will receive your 10% Garmin discount code in an email shortly afterwards.
I have a Nuvi 2555, which I'm trying to update via my Mac desktop.
This is my first GPS and my first map update, so I have no idea what I'm doing, other than following the instructions on Garmin Express.
I did at least do the backup before starting the update.