1、查找安装的yum，进行移除

rpm -qa | grep yum

rpm -e --nodeps xxxxxx

2、下载安装包

http://mirrors.163.com/centos/7.9.2009/os/x86_64/Packages/python-iniparse-0.4-9.el7.noarch.rpm
http://mirrors.163.com/centos/7.9.2009/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
http://mirrors.163.com/centos/7.9.2009/os/x86_64/Packages/yum-3.4.3-168.el7.centos.noarch.rpm
http://mirrors.163.com/centos/7.9.2009/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-54.el7_8.noarch.rpm

3、按顺序安装

rpm -ivh python-iniparse-0.4-9.el7.noarch.rpm
rpm -ivh yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
rpm -ivh yum-3.4.3-168.el7.centos.noarch.rpm yum-plugin-fastestmirror-1.1.31-54.el7_8.noarch.rpm

4、替换源

cd /etc/yum.repos.d

mv redhat.repo redhat.repo-backup

下载一份源文件进行修改，移至目录下 /etc/yum.repos.d/CentOS7-Base-163.repo

http://mirrors.163.com/.help/CentOS7-Base-163.repo

替换 $releasever 为版本号 7.9.2009

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-7.9.2009 - Base - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=7.9.2009&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/7.9.2009/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-7.9.2009 - Updates - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=7.9.2009&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/7.9.2009/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-7.9.2009 - Extras - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=7.9.2009&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/7.9.2009/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-7.9.2009 - Plus - 163.com
baseurl=http://mirrors.163.com/centos/7.9.2009/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

5 、清理、刷新缓存
yum clean all
yum makecache