The answer depends on if you're running GRUB2 or GRUB(1). Look at
$ apt-cache policy grub-common
If the 'Installed:' line shows a version close to 2.00 but less than 2.00 (1.99 or so), it's GRUB2 and I'd like to refer you to the excellent article in the Arch Linux wiki at https://wiki.archlinux.org/index.php/GRUB2
with important references at the bottom.
If not, you have GRUB 1 and will find instructions at https://wiki.archlinux.org/index.php/GRUB
Even if it's in the Arch Linux wiki, things should work alike on Debian.
If all these resources do not provide you with all you need to know, please post the output of the above command and your version of Debian, which you can find out by running
$ cat /etc/debian_version
which makes it easier to figure things out.