How to change battery in Ford Fusion key?

Turn the smart key upside down, then depress the notch near the top of the key and pull the back half off.

Remove the manual key from the holder, then prise the middle section of the smart key loose and the battery is underneath. To program a Ford Fusion smart key you need to close all the car doors, then open the driver's door, and press unlock on the driver's side door. Turn the ignition from OFF to RUN eight times in a row, finishing the 8th time on RUN. You need to do this step within 10 seconds of pressing unlock on the driver's door, and need to cycle the key within 10 seconds each time. When you have correctly entered programming mode the door locks will begin to lock and unlock as part of that cycle. Within 10 seconds of entering program mode, press any key on the smart key to be programmed.

Turn the key to OFF, the door locks will activate again to signal the end of programming.