Modify Grub2 Boot Menu

When booting the computer make note of which Menu Line is to be the default. Count from zero. (The first Menu Line is 0, the second is 1, etc..)

1. Open a Terminal session. Then using your favorite text editor, edit as superuser the grub configuration file. When finished, save and exit.:

   gksu aneditor /etc/default/grub

   So, if the fifth Menu Line is to be the default. Change the following line to read:

   GRUB_DEFAULT=4

   Choose a suitable delay to allow overriding at boot time – say 10 seconds. Change the following lines to read:

   #GRUB_HIDDEN_TIMEOUT=0
   GRUB_HIDDEN_TIMEOUT_QUIET=true
   GRUB_TIMEOUT=12

2. Now update Grub2 by issuing the following command:

   sudo update-grub

3. Reboot.

For more information please see: https://help.ubuntu.com/community/Grub2