What do you mean by "updating GRUB"? Update-grub is an Ubuntu command to re-generate the GRUB 2 configuration file. CentOS uses GRUB 1 a.k.a. "legacy GRUB", so there's no need for update-grub, grub-mkconfig or any other similar commands. You can edit the configuration file /boot/grub/grub.conf with any text editor.
I need to re-generate the GRUB configuration file to find out automatically boot entries from other linux OS, installed on the same machine. Anyway, if this doesn't work, I will edit the configuration file /boot/grub/grub.conf with a text editor.