Fixing Failed to Download Metadata for Repo 'AppStream' error

Below is a simple guide to fix Failed to Download Metadata for Repo 'AppStream' error
The whole thing is to replace the repo.

We start the terminal and spend 2 command:

 sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*

1	sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*

 sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-*

1	sed -i 's\|#baseurl=http://mirror.centos.org\|baseurl=http://vault.centos.org\|g' /etc/yum.repos.d/CentOS-*