Taking off a battery should be handled with immense care. Take your time and make sure not to puncture the outer case of the battery. This can be both harmful to you and your device.

Use the iFixit plastic pry tool in order to pry the battery up from the metal plate it's stuck to.

Make sure to work your way around the battery marked by the red box. Slowly and gently prying up until it is completely detached from the back plate.