Booting order

 Long press shift to display the grub booting menu.

sudo vim /etc/default/grub

Find the line that contains GRUB_DEFAULT=0 and set it to GRUB_DEFAULT=x where x is the index of grub menu item to which you would like to boot to by default. Note that the menu items are zero-indexed. That means that the first item in the list is 0 and that the sixth item is actually 5. So to boot to the sixth item in the list, the line would read GRUB_DEFAULT=5.

Additionally, if you want to use a kernel in the "Previous Linux Versions" menu, you'll want to change GRUB_DEFAULT=0 to GRUB_DEFAULT="2>x" (make sure to include the quotations), where x is the placement of the old kernel on the sub-list (assuming the "Previous Linux Versions" is third on the main list). Remember that the list always begins counting at 0.

Then build the updated grub menu.

sudo update-grub