grub2-reboot man page

grub-reboot [--boot-directory=DIR] MENU_ENTRY

grub-reboot sets the default boot menu entry for the next boot, but not further boots after that.  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