grub change default os
1. Find the string for the OS you want to set as the default with:
$ grep menuentry /boot/grub/grub.cfg
2. Highlight the string and copy it to the clipboard. (include the quotes.)
e.g. "Windows 10 (loader) (on /dev/sda1)"
3. Edit /etc/default/grub
$ sudo vi /etc/default/grub
4. Change the value of GRUB_DEFAULT from 0 to the OS string you copied and save.
5. Update Grub menu, by running the following command:
$ sudo update-grub
6. Reboot the computer to verify the expected OS is the default.