I rarely have to do this, so I can never recall exactly how on those odd occasions when it does come up. Usually it’s when one of the boot disks in a server has been replaced. Anyhow, as much to remind myself as for any other reason, here’s what I do on my CentOS servers which have /boot partitions as the primary partition on both /dev/sda and /dev/sdb.
# /sbin/grub grub> device (hd0) /dev/sda grub> root (hd0,0) grub> setup (hd0) grub> device (hd0) /dev/sdb grub> root (hd0,0) grub> setup (hd0)