Follow these steps to reset the password when using GRUB:
- Reboot the system, at the selection prompt, highlight the line for Linux and press ‘e’. You may only have 2 seconds to do this, so be quick.
- This will take you to another screen where you should select the entry that begins with ‘kernel’ and press ‘e’ again.
- Append single to the end of that line. Make sure that there is a space between what’s there and ‘single’. If your system requires you to enter your root password to log into single-user mode, then append init=/bin/bash after ‘single’. Hit ‘Enter’ to save the changes.
- Press ‘b’ to boot into Single User Mode.
- Once the system finishes booting, you will be logged in as root. Use passwd command and choose a new password for root.
- Type reboot to reboot the system, and you can login with the new password.