I will walk through the basic settings of GRUB on Ubuntu
You may be interested in

• Modify the default boot item
• Delete old grub items

1. Modify the default boot item
1. As root use your favorite editor to edit /boot/grub/grub.conf
2. Here you will find a line that says default followed by a number, the number is the number of the title entry that you want as default (counting from zero not one).
3. Change this number to the entry (OS) that you want (each OS has its own “title” line, count these starting at zero).
4. Save the file.
5. Reboot
2. Delete old grub items
1. Check your grub version

grub-install -v

2. If the grub version is prior to 1.96 then you can just edit the file /boot/grub/menu.lst and remove the unwanted entry.
3. If it is 1.96 and above you may not edit the /boot/grub/menu.lst directly. It is not allowed. In this case you have to delete the unwanted kernels manually and run the following command.

sudo update-grub