Q: How to change default Operating System boot-up for GRUB menu?

1. Read General Notes

2. sudo cp /boot/grub/menu.lst /boot/grub/menu.lst_backup
   sudo gedit /boot/grub/menu.lst

3. Find this line

   ...
   default         0
   ...

4. Replace with the following line

   default         X_sequence

5. Save the edited file (sample)