Use plastic prying tools to peel up the battery. Metal prying tools may puncture the battery and cause more damage. If you MUST use metal prying tools, go slow.

This process may take some time since the battery is held down with adhesive.

It is not really recommended to use the Heat Gun on the battery. However, the heat will loosen the adhesive. Therefore, if you decide to use a Heat Gun, apply the heat to the battery from underneath the device (heat the screen).