更换源之前,请确保Centos7 已经连上网
cd /etc
cd /etc/yum.repos.d/
mv CentOS-Base.repo CentOS-Base.repo.back #建议备份或者改名
wget -O CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
yum makecache #生成缓存
速度仍然提不上,则继续关闭ipv6:
https://jingyan.baidu.com/article/e5c39bf5cbc5d739d76033b1.html