Most likely, you just need to pop out the old disc, and put the new one in. If it doesn't work, turn the car off then back on, and if it still doesn't work, disconnect the battery for 10 minutes or so, then reconnect and turn back on. If the screen ever says anything about "loading" or something similar, just leave it alone until it's done starting up.
I'm not positive as to exactly where the Nav disc goes in your car, but check in the back hatch area under the carpet. It's a usually a black box that's about the same general size and shape of a CD changer, and is usually tucked into one side or the other.