To edit your grub.conf you will need to open a terminal, get root prevliges with su, and then run a text edit to modify the files, it should go something like this:
su
<enter you root password>
mount /boot
pico /boot/grub/grub.conf
<it may be /boot/grub.conf on your system>
Pico is a symple text editor that comes with most distros that I have used, you might also have 'nano' which is a pico clone. to save a file type 'ctrl+o', and to exit type 'ctrl+x'.
If you want to extend the time it waits at the selection screen, increase the value of 'timeout'.
You have two choices to change the default boot order. With my current setup my system defaults to Gentoo Linux 2.6.10. If I wanted it to default to Gentoo Linux 2.6.11-rc4 I could change the value of default to 1, or I could move the entire listing for 2.6.11-rc4 to be above the 2.6.10 entry. Changing the default setting is probably the best option.
default 0
timeout 4
splashimage=(hd0,0)/grub/splash.xpm.gz
title=Gentoo Linux 2.6.10
root(hd0,0)
kernel /vmlinuz-2.6.10 root=/dev/hda3 gentoo=nodevfs
title=Gentoo Linux 2.6.11-rc4
root(hd0,0)
kernel /vmlinuz-2.6.11-rc4 root=/dev/hda3 gentoo=nodevfs
If this doesn't work, let me know, and I'll to explain anything else I can. Also what distro are you using?