This question has come up a few times, so here's a consolidated how-to thread for battery replacement in the "emotion control" key.
DB9, Rapide, Vantage(?)
Experience thus far indicates no need to reprogram the key to the car, providing the key was working with a weak battery at the time the new battery was installed.
Replacement - Plastic Key
1) Turn the key over with the small hole facing up
2) Insert a small pin into the hole and push. The end cap should come off
3) The end cap has a directional clip on the end, make sure to put it back on the same way
4) With the end cap off, a slot is visible
5) Insert a screwdriver into the slot and twist in order to pry the 2 halves apart.
6) And here's the battery.
7) Assembly is simple, snap the 2 halves back together and then snap on the end cap.
Replacement - Crystal Key
Originally posted by MikeD http://www.6speedonline.com/forums/aston-martin/292992-dbs-sapphire-key.html#post3699855
Remove the two small screws on the underside at the end opposite the sapphire. (Actually, you need to bring the screws out only halfway.)
Slide the steel housing off and then prepare to slide the remote out.
TIP: The four buttons float between the remote and the plastic housing. They are not attached to the part you will slide out. It's better to hold the remote with the buttons facing down while you're working on it and to leave those buttons in place.
Be careful when sliding the remote out because you could tear the rubber cover of the remote if it gets hung up the any of the four buttons.