Quick video on popping open the Ford Keyless entry fob and replacing the battery. This specific one was for a Ford Mondeo 2010, but the fob is the same for a whole range of their models including Fiesta, Focus, Kuga and C-Max.

Amazon affiliate links:

Battery can be found @ https://amzn.to/33964ec

Screw driver can be found @ https://amzn.to/2riPZFw