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Do this by locating the small groove close to the charging port at the bottom of the device. This will let you hook your fingernail in the the back cover and pry it out. Once the back cover is off, you will have access to the phone's battery. Pry the battery out using your finger, making sure the battery is lifted from the bottom where there are no battery terminals. You will only need to briefly remove the battery, just enough that the metal connectors don’t touch anymore, and when you have, the phone's screen will shut off. Once the screen is dark, place back the battery into its compartment. Align the edges of the back cover properly and snap it back on. When the phone has been reassembled, press and hold the Power button until the phone vibrates briefly, signalling that the phone is starting up. The phone should then be responsive again upon reaching the home screen.

Summary:
Turn the phone over so its back is facing upwards. Remove the back cover. Remove the battery. Return the battery. Return the back cover. Turn on your device.