Both of these releases (RHEL5 and RHEL6) are at their End of Maintenance. RHEL6 is still within the Extended Lifecycle Subscription add-on phase, so if you have the ELS add-on, you can still get Critical and Important rated security errata for it. Without an ELS subscription, you'll not see further updates available for it.
what about the procedure to install security updates or CVEs on older redhat versions like redhat 5.8 or 6.10?
Both of these releases (RHEL5 and RHEL6) are at their End of Maintenance. RHEL6 is still within the Extended Lifecycle Subscription add-on phase, so if you have the ELS add-on, you can still get Critical and Important rated security errata for it. Without an ELS subscription, you'll not see further updates available for it.