To remove an iPhone case, remove all of the connecting screws. Before you begin working on the iPhone, it is recommended to touch exposed metal and get rid of static charges on your body.
Once you remove the screws, pry open the case with a small flathead screwdriver or a large suction cup. Always open the case slowly because the screen is still attached to the phone's components through several ribbon cables that you do not want to damage. To remove the case entirely, simply disconnect the cables, and finish prying the case away. Be sure not to lose the screws.