https://www.scientificlinux.org/news/sl6.4

Scientific Linux 6.4 is officially released for i686/x86_64

Scientific Linux 6.4 i386/x86_64 Mar 28, 2013

As a reminder, the SL6x repo always points to the most recent release. The
SL6x repo has been updated to SL6.4 at this time.

Users of the Scientific Linux 6x repo should run yum clean expire-cache
This should allow yum to notice the updated metadata within the 6x repo.