The error looks like-

And here goes the resolution (please carefully update/use the exact CentOS version you are using, in my case it was CentOS 6.9)

1. Go to /etc/yum.repos.d/ directory:

# cd /etc/yum.repos.d/

2. Make copy of original file:

# cp CentOS-Base.repo CentOS-Base.repo.old

3. Open and edit file with any text editor:

# vi CentOS-Base.repo looks

4. After editing it should looks like:

[base]
name=CentOS-$releasever - Base
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://vault.centos.org/6.9/os/$basearch
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
exclude=redhat-logos

5. Clean yum cache

# yum clean all

6. Run again yum update command:

# yum update

That’s It

An alternate Repo other than CentOS vault-

[base]
name=CentOS-$releasever - Base
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://ftp.uem.br/linux/CentOS/6.10/os/$basearch
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
exclude=redhat-logos

Ref: https://arstech.net/yumrepo-error-all-mirror-urls-are-not-using-ftp-https-or-file/