look in /etc/lilo.conf
find line "default = <name>", this sets the default kernel to start by label name.
change to "default = back1", save.
run "lilo -v -t" to test.
if all ok, install lilo: "lilo -v".
Btw, with the latest snafu (referring to the linux-2.4.15 fs corruption glitch) I opted out for automagical updates of core packages like kernel.