You can change the battery easily. Battery type CR 2032
Pull the keyfob apart, you may need to persuade the seperation with a small screwdriver. Buttons part opens up to reveal the battery, replace it. Ensuring the battery polarity is correct, (+ sign upwards) push them all back together. Test the result.
You get a minute or two to do this before you have to reprogramme I think.
I did the Focus one recently it worked a treat.