GRUB

GRUB is the bootloader that most of the UBO community uses now, instead of the original LILO. menu.lst is the plain text configuration page. The first section that looks like this is the default kernel, you can place as many alternatives as you like following the default:

title           Debian GNU/Linux, kernel 2.6.18-1-686
root            (hd0,0)
kernel          /boot/vmlinuz-2.6.18-1-686 root=/dev/hda1 ro
initrd          /boot/initrd.img-2.6.18-1-686
savedefault

Debian

To manually edit the list of kernels you can use upon boot:

# vi /boot/grub/menu.lst

and scroll to the bottom of this file.