Grub2 is using a new weird configuration format. If you want to change the default boot option you can find instructions here.

In short do a  fgrep menuentry /boot/grub/grub.cfg

find the menu entry you are interested in, edit /etc/default/grub  and copy the name of the entry in the

GRUB_DEFAULT=0
entry. Save the file and do a


sudo update-grub

Thats it