If you want to select which kernel or operating system is booted or to edit the kernel boot arguments the GRUB boot menu must be displayed and GRUB must display it for long enough for you to interrupt the automatic boot process. You must be root to make this change. The exact form of it depends on whether you're using GRUB 1 or GRUB 2:
Edit /root/grub/grub.conf and replace the line:
with the line:
By doing this you make GRUB display the boot actions menu and pause for 5 seconds to give you the chance to select an alternative kernel or edit the kernel boot arguments.
Edit the file /etc/default/grub and change the line starting with GRUB_TIMEOUT to increase the timeout value, which is the time boot menu will be displayed before the boot procress automatically loads the kernel at the top of the list. This time is measured in seconds, so changing the line to:
will cause the boot menu to be displayed for 5 seconds. If the line isn't present, add it as the first line in the file.