su to root
then
emacs /etc/lilo.conf

scroll down you should see something that says
default = "Linux"
and underneath that you should see three things like Linux, 2.4, 2.6 or somehting like that

to make 2.6 default boot, change where it says default = "Linux" to whatever the label of the 2.6 is.
Then save that
Then in the console type: lilo