Use something plastic to pry open the two halves. Start on the sides. I’ve marked the anchor points. The two on the right were the hardest to undo so be careful. I would try to pry open the other three first then if you can push against the tabs on right one at a time to free them. NOTE - the tabs are actually on the half not in the photo. The photo shows the points where the tabs go in to.
Image is before battery change. I cut the wires at the half way point and soldered the wires from the new battery to them. I just matched the wires black to black and red to red.