You change the battery in a Hyundai key fob by pressing the release button, removing the emergency metal key, and carefully opening the plastic fob housing. Insert a flathead screwdriver or the tip of the metal key into the small notch along the side seam to pop the two halves apart. Remove the old coin cell battery, usually a CR2032 size, and place the new battery inside with the positive side facing upward. Snap the plastic cover back together tightly and test the remote buttons to confirm it works.