How to change GRUB 2 boot order

First figure out which OS you’d like to be default. Observe its order number in the list of options (and note that GRUB starts counting at 0). Open a terminal and enter

sudo gedit /etc/default/grub

Change the GRUB_DEFAULT= to the number of the preferred OS. If my Ubuntu is third in the list (counting from zero, remember) I would set it to

GRUB_DEFAULT=3

Save, and then run

sudo update-grub

This should do it!