To conserve power, the iPhone automatically shuts itself off after a predetermined period of inactivity. Reducing this "Auto-Lock" period is a way to turn the iPhone off if the "Power" button isn't working. This "Power" button is on top of the device, and puts the iPhone in and out of sleep mode. The "Home" button can also bring the device out of sleep mode, but only the "Power" button can turn the device off.
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Launch the Settings application on your iPhone by tapping its icon on the home screen.
Tap the "General" button, followed by the "Auto-Lock" button.
Change the "Auto-Lock" setting to its minimum interval of "1 Minute" by tapping the relevant button.
Do not touch the screen or press any buttons when you've finished using the iPhone. After one minute, the "Auto-Lock" feature will automatically cut the iPhone off.