Access to the DVD player for the NAV system is behind a plastic cover on the passenger's side of the center console. Gently pry off the cover, and you'll see the Eject button on the player.
When I got the latest update for the NAV software, I made a backup copy just in case something happened to the original disk. You'll need a dual-layer DVD blank disk to hold the information on the backup, as it's more than a single-layer DVD+/-R can hold. I haven't put the backup copy in the DVD player, so I don't know if there are any compatibility problems or not.