How to change OS boot priority in Ubuntu

Login as root user.
edit the following file
/boot/grub/mentu.lst

you could see list of all installed OS here as follows:
Note: I have installed Ubuntu and XP in my machine

title Ubuntu 8.10, kernel 2.6.27-7-generic
uuid 1d034dd4-58c3-4ae9-abc0-c1b5e4591639
kernel /boot/vmlinuz-2.6.27-7-generic root=UUID=1d034dd4-58c3-4ae9-abc0-c1b5e4591639 ro quiet splash
initrd /boot/initrd.img-2.6.27-7-generic

title Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
uuid ....
kernel ....

title Ubuntu 8.10, memtest86+
uuid ....
kernel ....

title Other operating systems:
root ....

title Microsoft Windows XP Home Edition
root ....

The id start from 0.
i.e.
0 = Ubuntu 8.10, kernel 2.6.27-7-generic
1 = Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
2 = Ubuntu 8.10, memtest86+
3 = Other operating systems:
4 = Microsoft Windows XP Home Edition

The following line is taking care of which OS to boot by default

default 0

this means by default Ubuntu 8.10, kernel 2.6.27-7-generic will be booted

If we want to boot XP as default, change this line to

default 4

save the file. When you boot next time by default XP will be booted.