I'm experiencing some network problems with the new 2.6.21 kernel - can't really figure it out right now. It's quite unusable at the moment.
Luckily, I kept the previous kernel (2.6.18) installed, so I decided to revert back to it, until this issue is resolved. By revert I mean to say that I wanted it to be the default kernel that GRUB selects to boot.
It was late at night, so it took me two reboots before I actually looked at /boot/grub/menu.lst and saw the line specifying the default menu entry:
I guessed that menu entries are counted from 0 and up, so I modified the 0 to 2 (corresponding to the third menu entry, which is my old 2.6.18 kernel), and rebooted the machine.
I wonder how much futzing around it would've taken me before I actually read the manual. Or even read the comments in menu.lst (!).
Last time I acted this way was when I decided, late at night, to enlarge an NTFS partition, without a backup. I lost all my data. In the recovery process I managed to get my computer's motherboard fried. It's a long and sad story, which I'm not inclined to share.
Luckily, this time it just worked, so I had no need to search for the documentation ...