1) Switch your device off (long press the power button and select Power off).
2) Press and hold the Back and Home buttons (these are the two buttons that lie under the NVIDIA-logo Power button, on its left and right) while powering on your SHIELD Portable using the Power button. Once the Bootloader menu screen appears, you may let go of all three buttons.
3) You may use the Back buton to move up and Home button to move down. Select "Recovery mode" option from the Bootloader menu. Then press the Power button to confirm selection.
4) Your NVIDIA SHIELD Portable will reboot. After a few seconds, a "No command" message will appear on your screen.
5) Press the Power and Home buttons at the same time once. You should now be presented with an Android System Recovery menu. If you do not see this screen, press the Power and Home buttons at the same time again.
6) At the Android System Recovery screen, select "wipe data/factory reset". You may use "X" and "Y" buttons on your SHIELD to move up and down the menu. Press "A" to proceed once you have made a choice.
7) The recovery process should now begin. Please do not interrupt this process until it has completed and you have been prompted by Android's initial setup screen.