for example to 5.6:

Use the 5.6 repos from the vault in your yum configs:
http://vault.centos.org/5.6/

[base]
#name=CentOS-$releasever - Base
name=CentOS-5.6 - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
baseurl=http://vault.centos.org/5.6/os/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

usw....

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