Change Default Boot Options
After full installation, Ubuntu is set to be the default operating system to boot up if no key is pressed within a few seconds on a multi-boot system. You might want to set your preferred operating system to boot up by default. This can be done easily with StartUp-Manager.
Go to Applications (or Main Menu) > Accessories > Terminal.
Enter sudo apt-get install startupmanager (OR copy the highlighted code and, in the Terminal, press Ctrl-Shift-V to paste it).
Enter password used upon installation of Ubuntu.
Go to System > Administration > StartUp-Manager
Enter the same password to perform pre-configuration tasks, which include searching bootloaders to operating systems.
Select the default operating system from the pull-down menu, click "Close" to perform post-configuration tasks.
With StartUp-Manager, you can also do others such as manage Usplash themes, adjust bootloader menu resolution or set timeout in seconds. Avoid changing timeout to 0 seconds if you need to select a system to boot up from a multi-boot menu.