You load grub into a text editor with gksu gedit to load the text editor
with root privileges. (Server may not have this in which case use nano
instead of gedit.) Then you navigate the text editor to /boot/grub in the
root file system of your server edition. Then you open the file called
menu.lst. Scroll down to the line that says default 0. Grub uses ordinals
(starts from zero). This means that if you don't press any keys it will boot
the first OS in the list. You can change that number to anything else in
your list. Just remember that it counts in ordinals.

You can do this in one step with: gksu gedit /boot/grub/menu.lst

You can edit grub from any OS that you can boot into. It does not require
that you work from server. Just remember to work on grub in the last OS that
you installed which is likely server since that is the choice that grub
gives you by default. It is easier to launch a root nautilus to change
partitions then navigate to /boot/grub and right click on menu.lst and
choose open in text editor. To get a root nautilus you use gksu nautilus in
a terminal. Do not close the terminal window until you are finished or it
will close nautilus or gedit.

Instructions are provided here:
https://help.ubuntu.com/community/GrubHowto/ChangeDefaultOS
You can learn more about grub here:
https://help.ubuntu.com/community/GrubHowto