1、进入到yum源相应的目录
cd /etc/yum.repos.d/
2、创建备份目录，把相应的文件进行备份
mkdir bak
mv * bak/
3、编辑配置CentOS-Base.repo文件

[root@localhost yum.repos.d]# cat CentOS-Base.repo 
# file: /etc/yum.repos.d/CentOS-AppStream.repo
[aliyun-AppStream]
name=CentOS-$releasever - AppStream
baseurl=http://mirrors.aliyun.com/centos/$releasever/AppStream/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
# file: /etc/yum.repos.d/CentOS-Base.repo
[aliyun-BaseOS]
name=CentOS-$releasever - Base
baseurl=http://mirrors.aliyun.com/centos/$releasever/BaseOS/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

[aliyun-extras]
name=CentOS-$releasever - Extras
baseurl=https://mirrors.aliyun.com/centos/8.0.1905/extras/x86_64/os/
enabled=1
gpgcheck=1

4、安装epel-release源
dnf -y install epel-release
5、清yum缓存，重新加载

dnf clear all
dnf makecache
dnf repolist