" That information is Classified! "
Seriously though, if there *is* such a GUI, it would be called something like 'Configure boot' or 'Boot configure' or LILO configure or 'GRUB configure' or similar.
The actual file would be lilo.conf (using LILO) or grubmenu.lst or menu.lst (on GRUB systems).
on LILO, setting the default is easy, but I am unfamiliar with GRUBs way of doing things.