Selecting default OS or kernel in GRUB boot loader

If you are wishing to change the default OS or kernel in GRUB boot loader, you can easily make the changes

# vim /boot/grub/grub.conf

Change the  default=0 to 1 of other numeric if that is the kernel or OS you wish to use.

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You do not have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /, eg.
#          root (hd0,0)
#          kernel /boot/vmlinuz-version ro root=/dev/sda1
#          initrd /boot/initrd-version.img
#boot=/dev/sda
default=1
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-238.19.1.el5.centos.plus)
        root (hd0,0)
        kernel /boot/vmlinuz-2.6.18-238.19.1.el5.centos.plus ro root=LABEL=/ rhgb quiet
        initrd /boot/initrd-2.6.18-238.19.1.el5.centos.plus.img
title CentOS (2.6.18-238.19.1.el5)
        root (hd0,0)
        kernel /boot/vmlinuz-2.6.18-238.19.1.el5 ro root=LABEL=/ rhgb quiet
        initrd /boot/initrd-2.6.18-238.19.1.el5.img