When your Ubuntu system boots, you will see the GRUB menu if you hit the Esc key, or if you’ve enabled the menu to show by default. The only issue with this is that the default timeout is only 3 seconds. You may want to increase this amount… or you may even want to decrease it. Either one is simple.
Open up the /boot/grub/menu.lst file in your favorite text editor. I’m using gedit:
sudo gedit /boot/grub/menu.lst
Now find the section that looks like this:
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined).
The timeout value is in seconds. Save the file, and when you reboot you will have that many seconds to choose the menu item you want.