CentOS8

sudo sed -e 's|^mirrorlist=|#mirrorlist=|g' \
         -e 's|^#baseurl=http://mirror.centos.org/$contentdir|baseurl=https://mirrors.ustc.edu.cn/centos|g' \
         -i.bak \
         /etc/yum.repos.d/CentOS-Base.repo \
         /etc/yum.repos.d/CentOS-Extras.repo \
         /etc/yum.repos.d/CentOS-AppStream.repo

CentOS7 && CentOS6

sudo sed -e 's|^mirrorlist=|#mirrorlist=|g' \
         -e 's|^#baseurl=http://mirror.centos.org/centos|baseurl=https://mirrors.ustc.edu.cn/centos|g' \
         -i.bak \
         /etc/yum.repos.d/CentOS-Base.repo

update cache

yum makecache

epel

sudo yum install -y epel-release
sudo sed -e 's|^metalink=|#metalink=|g' \
         -e 's|^#baseurl=https\?://download.fedoraproject.org/pub/epel/|baseurl=https://mirrors.ustc.edu.cn/epel/|g' \
         -i.bak \
         /etc/yum.repos.d/epel.repo