Do you mean the key fob battery? Look on the back of the key fob, the battery is a CR2032. You can buy them pretty much anywhere that sells watch batteries. Try your local drug store, convience store, grocery store, or places like Walmart and Target.
Is just changing the battery enough? Does it need to be reprogramed?
I just checked the owners manual and it doesn't mention anything about reprogramming, so I would presume it's plug and play of sorts. It also says the battery should last 4 years too....but I would think that would depend on how often it's used.