Just work your way around the edge horizontally with a plastic pick. Watch out, because the plastic is soft, and the plastic ears are thick - it's hard to open it up without damage. Once you clicked them all out, you'll have to apply some continuous force, as the battery has a nice double sided tape sticking it both to the bottom and top part of the case, BUT also the antennae are glued to the case - make sure you don't rip them.
Hopefully you won't need to open it, though.