The standard linux system use grub (Grand UNified Bootloader) to manage its booting process. To change the boot order in linux, there is one file that you need to change which is /etc/default/grub.
To check which number your operating system resides, just run grub-reboot, and press double tab after the command to get the list. The list started from 0, so if your operating system of choice is at location 3, the number is 2.
1. Open /etc/default.grub with
sudo nano /etc/default/grub
2. Change this line to suit your need
GRUB_DEFAULT=2
3. Save and exit
4. Update grub
sudo update-grub
That's all, try rebooting your machine and see if grub actually follow the configuration that you have setup.
