1. check the kernel that has installed in the os :

[root@localhost centos]# awk -F\' '$1=="menuentry " {print $2}' /boot/efi/EFI/centos/grub.cfg
CentOS Linux (5.4.32) 7 (Core)
CentOS Linux (3.10.0-957.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-0d7254b68edc46d99a89bd8ad0818cf7) 7 (Core)

2.查看此时默认的启动项

grub2-editenv list命令查看系统当前的默认内核
CentOS Linux (3.10.0-957.el7.x86_64) 7 (Core)




3.设置自己的需要的启动项




使用命令grub2-set-default 'centOS Linux (5.4.32) 7 (Core)' 
done




4.reboot