Use the flat end of a spudger to press and hold the small gold 'battery disconnect' button.
If the power LED is lit up, continue holding the button until the LED goes dark, and then release. This may take up to 10 seconds.
If the LED does not light, release the button after 5-10 seconds. Press and hold it again for 5-10 seconds, and release. Finally, press and hold it a third time for 5-10 seconds, and release.