When I say

# yum update

This is output:

Loaded plugins: fastestmirror, replace, security

Setting up Update Process

Loading mirror speeds from cached hostfile

YumRepo Error: All mirror URLs are not using ftp, http[s] or file.

 Eg. $releasever is not a valid and current release or hasnt been released yet/

removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/$releasever/base/mirrorlist.txt

Error: Cannot find a valid baseurl for repo: base

update

This is my base repo:

# cat CentOS-Base.repo 

# 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-$releasever - Base

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra

#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6



#released updates 

[updates]

name=CentOS-$releasever - Updates

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra

#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6



#additional packages that may be useful

[extras]

name=CentOS-$releasever - Extras

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra

#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6



#additional packages that extend functionality of existing packages

[centosplus]

name=CentOS-$releasever - Plus

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra

#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/

gpgcheck=1

enabled=0

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6



#contrib - packages by Centos Users

[contrib]

name=CentOS-$releasever - Contrib

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib&infra=$infra

#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/

gpgcheck=1

enabled=0

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6

That's probably because the CentOS version you're using reached end-of-life, which means that no new updates will be released by Red Hat, so the tree has been moved to vault.centos.org, and the baseurl= links in the /etc/yum.repos.d/CentOS-Base.repo files should refer to that link.

Please see
https://unix.stackexchange.com/a/359927/112443