Grub2: Set default boot option

o Get the string of the menu entry you desire to be default

  fgrep menuentry /boot/grub/grub.cfg
  ...
  menuentry "Some OS" {
  ...
 
o Add menu entry to /etc/default/grub
  
  vi /etc/default/grub 
  GRUB_DEFAULT="Some OS"

o Update grub

  update-grub