First you need to identify number of OS that needs to be loaded
cat /boot/grub/grub.cfg |grep class |grep '(' |grep -n menuentry
1:menuentry 'Ubuntu, with Linux 3.2.0-63-generic-pae (recovery mode)' –class ubuntu –class gnu-linux –class gnu –class os {
2:menuentry 'Ubuntu, with Linux 3.2.0-63-generic (recovery mode)' –class ubuntu –class gnu-linux –class gnu –class os {
3:menuentry 'Ubuntu, with Linux 3.2.0-61-generic-pae (recovery mode)' –class ubuntu –class gnu-linux –class gnu –class os {
4:menuentry 'Ubuntu, with Linux 3.2.0-61-generic (recovery mode)' –class ubuntu –class gnu-linux –class gnu –class os {
5:menuentry 'Ubuntu, with Linux 3.2.0-58-generic (recovery mode)' –class ubuntu –class gnu-linux –class gnu –class os {
6:menuentry "Ubuntu, with Linux 2.6.32-51-generic (on /dev/sda1)" –class gnu-linux –class gnu –class os {
7:menuentry "Ubuntu, with Linux 2.6.32-51-generic (recovery mode) (on /dev/sda1)" –class gnu-linux –class gnu –class os {
8:menuentry "Ubuntu, with Linux 2.6.32-28-generic (on /dev/sda1)" –class gnu-linux –class gnu –class os {
9:menuentry "Ubuntu, with Linux 2.6.32-28-generic (recovery mode) (on /dev/sda1)" –class gnu-linux –class gnu –class os {
Please note that calculation in grub starts from the zero, so for example number of
7:menuentry "Ubuntu, with Linux 2.6.32-51-generic (recovery mode) (on /dev/sda1)" –class gnu-linux –class gnu –class os {
is 6
Update the line GRUB_DEFAULT=0 into the followin file
sudo vi /etc/default/grub
from
GRUB_DEFAULT=0
to
GRUB_DEFAULT=999
where 999 is number of partition that needs to be loaded first
Update your grub.cfg… Read the rest