I've never opened up one of those, but if it's centre seam with the case in two roughly equal parts, some of those use tongues latching the two halves at the seam. Try prising one side out and away from the other along on of the long sides and see it that releases at that point. You may need to prise each half in turn to find the one willing to open outwards. If successful you can work around the case to separate it completely. Don't worry about breaking the latches, the case screws will hold it back together afterwards as I've found with similar batteries previously.