How to Change the Default Boot Operating System with Ubuntu GRUB 2

1.) cat /boot/grub/grub.cfg see the order of the wanted kernel. Starts from 0.
2.) gksu gedit /etc/default/grub change GRUB_DEFAULT=0 value to wanted kernel
3.) run sudo update-grub to update
4.) reboot and check with uname -r to see if correct kernel selected.