CentOS 7 update using "yum update" does not work

Run the following command to clean the metadata:

yum clean all

This will clean all yum caches including cached mirrors of your yum repositories. On the next run it will get a new list of mirrors.

The CentOS-Base.repo file in /etc/yum.repos may have changed since you installed. Current mirror list is as below, with the baseurl= line commented out.

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