The Nissan website points you to this link for Nav updates
NAVTEQ - Maps for your Nissan Navigation System
$129 for the activation code which the dealer uses to install the update -- for a fee of course
You should see if it takes you where you want to go before you pay for an update -- after all, you already know where your street is.
I wouldn't worry about a screen protector for the Nav. They seem like tough screens not a blemish on mine after 6 months.