If the boot loader isn’t locked, after GRUB shows: Hit ‘e’ to edit the boot entry. Append this to the end of the linux line:

 init=/bin/sh

You should then have a ‘# ‘ prompt. The root filesystem will be mounted read-only. Remount the file system in read write mode by entering:

mount -o remount,rw /

Change the ‘root’ account password:

passwd root