Use a prying tool to pry open the key fob casing.

Tools other than a screwdriver can be used, but for safety purposes it is important to use a tool that is small enough to open the seam but not sharp enough to hurt yourself.

While it does take a somewhat large amount of force to open the case, it should not take more than you should be able to provide with just a tool. Too much force may damage the components, so be sure to be careful when prying.