Fedora

• Press ESC at the GRUB prompt.
• Press e for edit. Highlight the line that begins kernel or linux press e 
• Go to the very end of the line and add

•  rw init=/bin/bash 

• Press Ctrl+X to boot or Press Enter, then press b to boot your system. 

mount -o remount,rw /
passwd root 
touch /.autorelabel
sbin/reboot -f

Ubuntu 

mount -o remount,rw /
passwd root
exit

General 

passwd -d root # to delete