Its a standard coin cell. These are held in by the white plastic holder. You have to carefully apply pressure to coin cell opposite of the side you will apply outer pressure to the white plastic to allow for the coin cell to tip up in the corner that the white plastic is pushed carefully out away from coin cell.
If you apply too much pressure you can break the plastic holder and be in trouble though so only apply the bare minimum pressure to get the coin cell top pop up out of the holder.
Putting replacement battery back in you need to put the new coin cell (probably a 2032) battery in the same polarity orientation.