Assuming they are all the same, I just flip the lid off the fob (flick the metal key out and pull/pry the exposed ridge of the cover), pop out the old one and slip in the new one, and press the lid back on.
Mine is like this one...
https://www.youtube.com/watch?v=uxR6FvgiVeo
There is no coding it just works, but probably best to do it within 5 minutes, not take out the battery and then order one and wait for it to come. It is a CR2032