The battery ran out on my key fob for my 2014 Mazda6 and I had to replace it. I thought I’d make a video to demonstrate how to do it, since I had to figure it out.
Interestingly, my car wouldn’t start when the battery died, since the car must sense the key inside in order to turn on via the push-start button ignition. Otherwise, people could just break in and push the button to steal the car and drive off.
I did learn later that I can hold the fob up to the push-start button and a little sensor will allow it to turn on.
Either way, here is the video for how to take the key fob apart and replace the battery.