This post describes how to change the default startup choice while booting up CentOS 7.
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 /boot/grub2/grub.cfg
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.