Press Power Button for about 3 seconds. Count 1001, 1002, 1003,1004. Release the Power Button. NUC should reboot into Power Button Menu. You should release the Power Button before 4-sec shut down override.
Press F4 and the recovery shall start (it can take up to 30 seconds for messages to appear on the screen).
When the recovery finishes, press on Power Button and hold it, until NUC switches OFF. Pull out the power cord.
Replace the power cord and press Power Button to switch the computer to ON.
Enter Bios setting by pressing F2 during boot process.
Press F9(followed by "Y"), to set Bios to default settings. Press F10 (followed by "Y"), to save the settings and exit to O.S. Let the NUC fully reboot.
If before update, you have customized bios settings, you can enter again to Bios setting to change the necessary settings.