You cannot "download" any map to the unit's memory.
If you want more than what the unit has, you have to purchase a Navionics card that has the coverage you need.
The Navionics+ Preloaded card has all the maps for the US and Canada.
The current maps can be viewed on the Navionics website's Web App.
What you see is what can be on the card when you do an update of the card.
You can also download the Sonar Charts map layer when you do an update.