Hello,
Our environment contains VCenter 5.5 with external SSO for each VCenter, we have a plan for migrating the VCenter to External PSC HA for VCSA (6.5)
Let me know the steps are KB releases from VMware.
Have you done any research on your own? This process is very well documented at this point with multiple articles and KBs.
Please refer to doc Example Upgrade Paths for vCenter Server version 5.5 to version 6.5
