Change the default startup choice while booting up CentOS 7.

grub2-mkconfig grub

Edit the file /etc/default/grub

Change the line



GRUB_DEFAULT="Windows 7 (loader) (on /dev/sda1)"

The value to add is determined by looking at the menuentry lines in makeactive 

Choose the menuentry that you would like to be default.

To make this change active run the command

# grub2-mkconfig -o /boot/grub2/grub.cfg

You will see this change the next time you reboot.

Дополнительно здесь: