grub command line change default boot code example

Example: change default boot grub

sudo cp /etc/default/grub /etc/default/grub.bak
#create a backup if you want
sudo nano /etc/default/grub
# find the line that contains GRUB_DEFAULT=0
# change it to what you want. 0 is normally Ubuntu, 1 is advanced options.
sudo update-grub
reboot