Those who enjoy hacking their Android smartphones and have installed a custom recovery option have complete control over the ROM their phone runs. This means that you can download optimized ROMs from the Internet and flash them to your phone, potentially improving your Android device's usefulness. You can learn how to flash an update.zip file to your Android smartphone by following a few steps.
Power down your Android phone.
Press and hold the volume up button, the home button and power button simultaneously until the custom recovery screen appears.
Push the volume down button to move down one menu option, then tap the "Call" button on your phone to select the "Apply Update From SDcard" option.
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Move down again using the volume down button and then press the "Call" button to select "Yes - Install /sdcard/update.zip." This will flash your Android phone with the update.zip file stored on your SD card.