To change the battery in a Honda key fob, you will need a small flathead screwdriver and a replacement battery. First, use the screwdriver to gently pry open the key fob by inserting it between the two halves of the fob and twisting it to separate the two halves. Once the fob is open, you should see the battery inside. Use the screwdriver to carefully remove the old battery from the key fob. Then, take the new battery and insert it into the key fob, making sure that the positive and negative terminals are aligned properly. Once the new battery is in place, reassemble the key fob by pressing the two halves together until they snap into place.