修改GRUB改变启动顺序是作为一个Linux运维人员必备的技能，特别是内核升级失败的情况下。
CentOS官方的文档讲解的很详细，可以参考一下
在 CentOS 7 上设置 grub2
下面是一个老外的博客上面写的，也可以实现，就是自己修改默认的GRUB的值
Change default grub startup in CentOS 7
This post describes how to change the default startup choice while booting up CentOS 7.
Edit the file /etc/default/grub
Change the line1
GRUB_DEFAULT=saved
to1
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 command1
# grub2-mkconfig -o /boot/grub2/grub.cfg
You will see this change the next time you reboot.
