CentOS 7

Edit /etc/default/grub to your liking.

Make changes effective with grub2-mkconfig.

grub2-mkconfig -o /boot/grub2/grub.cfg

UEFI systems use a different path.

grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg

CentOS 6

Edit /boot/grub/grub.conf.