Yum update centos 6.x after EOL

***Backup the original file

mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo-backup

*** Create the yum upgrade CentOS-Base.repo file that use vault.centos.org or mirrors.sohu.com

vi /etc/yum.repos.d/CentOS-Base.repo

[base]

name=CentOS-6

failovermethod=priority

baseurl=https://vault.centos.org/6.9/os/x86_64/

#baseurl=http://mirrors.sohu.com/centos/6.9/os/x86_64/

gpgcheck=0

*** Then clean up yum cache & rebuild again

yum clean all & yum makecache

*** Try yum upgrade again

yum upgrade

*** God bless you.