How to do it...

In this section, we will cover the steps involved in upgrading a vSphere 5.5 environment to vSphere 6.5:

Backup the current configuration: Take snapshots of SSO, vCenter, and database VM before you start the upgrade. Also, take backups of the database if vCenter is running on a physical machine and using an external database.
Upgrade SSO servers to vSphere 6.5 PSC: Regardless of the platform (Windows or vCSA), the Single Sign-On component servers should be upgraded from 5.5/6.0 to vSphere 6.5 before the vCenter upgrade.
Upgrade vCenter to VCSA 6.5: For instructions on how to migrate from Windows to VCSA 6.5, read the section Upgrading vCenter Server - Migrating from Microsoft Windows to VCSA of this chapter. Single Sign-On and other services will be migrated. vCenter 6.5 can also be installed on a Windows Server, so upgrading vCenter can also be performed without having to rebuild a new machine. Read the section Upgrading vCenter Server on Microsoft Windows for instructions. In either case, a database upgrade will be performed.

VCSA 6.5 no longer supports the use of an external database. Hence, the current database will be migrated to a PostgreSQL database.

Upgrade vSphere Update Manager: VUM will be upgraded and made part of the vCenter Server if the current vCenter system being upgraded also has VUM installed on it. If VUM is installed on a separate machine, which is mostly the case in enterprise infrastructures, then you will need to run the vCenter Migration Assistant on the VUM machine as well.
Use vSphere Update Manager to upgrade the hosts to ESXi 6.5: Read the Chapter 14, Upgrading and Patching using vSphere Update Manager for instructions on how to use VUM to upgrade ESXi hosts by scheduling upgrades/updates.
Use vSphere Update Manager to upgrade the virtual machine hardware and VMware tools: Read Chapter 14, Upgrading and Patching using vSphere Update Manager, for instructions.