First on current RHEL5 backup the list of packages installed just in case if you want to cross verify.
rpm -qa --queryformat "%{NAME}\n" | sort > /root/all_old_packages
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Check Selinux is enabled or not if enable disable it and uninstall it .
command used is
sestatus
SELinux status: disabled ( in my case it is disabled )
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vi /etc/inittab change to runlevel 3
id:3:initdefault: ( mine is a server which is already in runlevel 3 )
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remove old docs
rm -rf /usr/share/doc/HTML/*/docs/common ( this is mandatory else upgrade will fail)
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Now boot with RHEL6 DVD on boot menu press esc
then enter " linux upgradeany "
Follow instructions and you are done . IF you face any issues you can got to other run levels and check what caused failure. You can troubleshoot that issue and upgrade. During my installation i faced only one issue . That is the reason i have deleted old documents.