You can use the GUI installer to perform an interactive upgrade of a vCenter Server Appliance 5.5 or 6.0 that uses an external vCenter Single Sign-On or Platform Services Controller instance to vCenter Server Appliance 6.5 with an external Platform Services Controller. You must run the GUI upgrade from a Windows, Linux, or Mac machine that is in the same network as the appliance that you want to upgrade.

Upgrade Workflow of a vCenter Server Appliance with an External Platform Services Controller
Upgrade workflow that consists of two stages

See Prerequisites for Upgrading the vCenter Server Appliance or Platform Services Controller Appliance.

See Required Information for Upgrading a vCenter Server Appliance 5.5 or 6.0 or Platform Services Controller Appliance 6.0.

Upgrade or migrate the external vCenter Single Sign-On 5.5 or Platform Services Controller 6.0 instances in the domain to Platform Services Controller 6.5.