I imagine you could manually edit the GRUB bootloader file and
accomplish your desired boot order, however, I've never needed to do
this personally, since my needs are met when all the options are
presented regardless of their order (I have no pesky secondary users to
deal with).

You could edit the file using the GEDIT utility, but wait until you have
received advice from someone with plenty of experience in this area (I
am disqualified immediately). If you were to mangle the editing, it
would likely render GRUB useless, causing the family to beat you
senseless with blunt objects as the cursor blinked maddeningly against
the cold, black background of death