To change the battery in a Honda key, you will need to locate the small button on the back of the key. Once you have found the button, press it to release the key blade. Then, use a small flathead screwdriver to gently pry open the key fob. Once the key fob is open, you should see the battery inside. Use a small Phillips-head screwdriver to remove the battery from the key fob. Then, replace the old battery with a new one of the same size and type, making sure to insert it in the correct orientation. Finally, reassemble the key fob and test the new battery to make sure it works properly.