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
Save, and then run
This should do it!