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First, boot into recovery mode.
Restart your computer.
Wait until the UEFI/BIOS has finished loading, or has almost finished. (During this time you will probably see a logo of your computer manufacturer.) Note: UEFI fast boot may be too fast to give time to press any key.
With BIOS, quickly press and hold the Shift key, which will bring up the GNU GRUB menu. (If you see the Ubuntu logo, you've missed the point where you can enter the GRUB menu.) With UEFI press (perhaps several times) the Escape key to get grub menu.
Select the line which starts with "Advanced options".
Select the line ending with "(recovery mode)", probably the second line, something like:
Ubuntu GNU/Linux, with Linux 3.8.0-26-generic (recovery mode)
Press Return and your machine will begin the boot process.
Open a terminal window by pressing Ctrl + Alt + T keys on the keyboard.
Disable Secure Boot in shim-signed:
sudo mokutil --disable-validation
Enter a temporary password between 8 to 16 digits (e.g. 12345678).
Enter the same password again to confirm.
Reboot the system:
Press any key when you see the blue screen (MOK management)
Select Change Secure Boot state
Enter requested characters of the password you had selected, and press Enter.