default (entry number of your preferred OS)
where the entry number of your preferred OS is the line that your OS appears on in the Grub boot menu, minus one, because the numbering of entries starts at 0.
It is always good practice to save system files before editing them. With this in mind, you should save a copy of your menu.lst before editing so that you can restore it to a known working state in case you break it. You can do this with a simple
cp menu.lst menu_copy.lst
from the command line