If it's the same as our UK model then remove the emergency key from the fob housing, then, working inside the hole where you just removed the key - use a screwdriver to prise the two sections of the casing apart, sliding the screwdriver towards the other end as space permits. It takes one CR2025 battery. The procedure is in our owners manual, perhaps yours is the same.
Were you able to replace it? if so, I just replaced mine and it's a breeze. 1) Unclip manual key from underside. 2) use a credit card/any plastic card and/or tiny screwdriver and twist gradually between the casing of the key. 3) remove the battery shield. 4) Place battery + up. 5) reverse order to complete installation.