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Make sure your device has no less than 50% of it's battery. Locate the power button on your device and press it to turn off. Depending on the device you may need to press and hold until your screen asks you to confirm shut down. This will boot your device into a warning screen. " This enables you to enter download mode.

Summary:
Checking battery life. Shut the device down. Hold the "Volume Down," "Home," and "Power" button at the same time. Press "Volume Up.