# cat /etc/redhat-release
Scientific Linux release 6.4 (Carbon)

# yum clean all
Loaded plugins: fastestmirror, security
Cleaning repos: xxxx xxxx sl sl-security
Cleaning up Everything
Cleaning up list of fastest mirrors

# yum --releasever=6.5 update sl-release
・・省略・・
Complete!

# yum update
・・省略・・
Complete!

参考:
Scientific Linux – How To Upgrade To The Latest S.L. 6.x Release