How do you change a Mercedes key battery?

Depends on the year. The owners manual should show you how to pull the back of the key fob (that holds the batteries. Most of the key fobs take two batteries. Make sure you keep the old ones to make sure you get the right size. Once you have the batteries back into the key fob you will need to put your key in the ignition so the key functions will work again. If your light doesn't blink with the new batteries you installed them backwards *I did that*