1.First,Remember which system need to boot at first.if the windows order is 3
2.backup grub file before we edit it.
#cd /etc/default
#sudo mv grub grub.back
#edit this line
GRUB_DEFAULT="0"
change it to GRUB_DEFAULT=saved
3.sudo update-grub
Default boot order,
4.sudo grub-set-default 2
