Back up old configurations:

List all currently configured repositories and move CentOS and epel configurations afterwards:

[root@server ~]# ls /etc/yum.repos.d/
CentOS-Base.repo  CentOS-CR.repo  CentOS-Debuginfo.repo  CentOS-fasttrack.repo  CentOS-Media.repo
CentOS-Sources.repo  CentOS-Vault.repo  CentOS-x86_64-kernel.repo  epel.repo  epel-testing.repo

[root@server ~]# mkdir /etc/yum.repos.d/old
[root@server ~]# mv /etc/yum.repos.d/CentOS*.repo /etc/yum.repos.d/old/
[root@server ~]# mv /etc/yum.repos.d/epel*.repo /etc/yum.repos.d/old/

Create new repository configurations:

[root@server ~]# cat /etc/yum.repos.d/CentOS.repo

[base]
name=CentOS-7.9.2009 - Base
baseurl=http://vault.centos.org/7.9.2009/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

#released updates
[updates]
name=CentOS-7.9.2009 - Updates
baseurl=http://vault.centos.org/7.9.2009/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

# additional packages that may be useful
[extras]
name=CentOS-7.9.2009 - Extras
baseurl=http://vault.centos.org/7.9.2009/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
metadata_expire=never

# additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-7.9.2009 - CentOSPlus
baseurl=http://vault.centos.org/7.9.2009/centosplus/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=0
metadata_expire=never

#fasttrack - packages by Centos Users
[fasttrack]
name=CentOS-7.9.2009 - Contrib
baseurl=http://vault.centos.org/7.9.2009/fasttrack/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=0
metadata_expire=never

Add EPEL repository configuration

This step is optional and only needed if EPEL has been used before:

[root@server ~]# cat /etc/yum.repos.d/epel.repo

[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
baseurl=https://archives.fedoraproject.org/pub/archive/epel/7/$basearch
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
metadata_expire=never

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
baseurl=https://archives.fedoraproject.org/pub/archive/epel/7/$basearch/debug
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1
metadata_expire=never

[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
baseurl=https://archives.fedoraproject.org/pub/archive/epel/7/SRPMS
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1
metadata_expire=never

Load new packet sources

Remove all currently available metadata: 

yum clean all

Load a new list of all available packages and to check if your local installation has all available updates. 

yum check-update