grub-set-default — Set the default boot menu entry for GRUB.
grub-set-default [--boot-directory=DIR] MENU_ENTRY
grub-set-default sets the default boot menu entry for all subsequent boots. This command only works for GRUB configuration files created with GRUB_DEFAULT=saved in /etc/default/grub.
--boot-directory=DIR
Find GRUB images under DIR/grub. The default value is /boot, resulting in grub images being search for at /boot/grub.
MENU_ENTRY
A number, a menu item title or a menu item identifier.
info grub