For some people who need clearer instruction.. take note of root external SD card
How to install padfone 2 firmware?
First, make sure you have the firmware (*.zip) file in the root of an external SD card.
Also, make sure that the Padfone 2 is fully charged, and connected to a power source.
If you're downgrading or changing the firmware region by using a full firmware package, we would recommend using the wipe feature before you apply the update.
1. Undock the Padfone 2.
2. Power off the Padfone 2.
3. Hold down the volume down button, keep holding it, and press the power button. Now you will enter the first menu.
4. Press the power button to enter "Recovery mode".
5. Select the second option to install from an external SD card.
6. Wait for the installation to complete, depending on the update, your device will reboot automatically, or you
have to select the reboot option manually when it appears.