Turn the phone completely off. To do this, either manually hold down the power button, or remove the battery.

To remove the battery: At the top corner of your phone, put the nail of your index finger in the seam between the screen frame and the back cover. Press the middle of the back cover, and bend the cover to loosen the top hooks. Remove the cover and lift the battery out.

Do not use any sharp tools, because they will damage the screen.