It depends if You are using grub or grub2 ... I'll assume You are using grub (the "old" one) ... In that case in file /boot/grub/menu.lst change the line "default 0" to "default x" where x+1 is the order of OS in the grub list (the order You see at boot i.e. the order in which they are listed in aforementioned file) You want to use.
Originally Posted by James Dee
Be careful since if You mess up that file You could be left out of Your computer for a while before You master using LiveCd to regain control ...
To edit that file You could use
I'm a bit rusty on using grub so take this with a grain of salt ...
sudo nano /boot/grub/menu.lst