The battery in the key fob is a Panasonic CR2032. You can buy them cheap on EBay.
The replacement steps are very easy (don't go to the dealer):
1. Remove the valet key by pushing on the slide and pulling it straight out.
2. Using a thin-bladed screwdriver or a knife edge, unsnap the key fob by
gently prying around all of the edges.
3. One the key fob splits apart, simply pull out the old battery and insert the
new one. (Don't worry, unlike some watches, there are no loose parts
inside).
4. Handle the new battery around the edges and push it into place using
your thumb and index finger (Tip- Use a small piece of paper towel, or rubber gloves, to
avoid getting any hand oil on the new battery and angle the battery in
towards the silver retaining clips).
5. Align the two halves of the key fob and snap them together.
6. Reinsert your valet key.
7. Test your locks to insure that they are working.
You are done (less than 5 minutes)!