GRUB bootloader starts up what's necessary for your Linux or UNIX system to boot up. You can edit its settings, like various boot options and which operating systems to select from, by editing the the /boot/grub/grub.cfg or /etc/grub.conf depending on your system. Graphical programs are also available for this purpose. See our GRUB Boot Loader overview for more.
Once you've edited your configuration you'll need to update grub to use it. This is very easily done by this single command:
$ sudo update-grub
Then once you reboot your new config should be active.