This first task to perform during the Virtual SAN upgrade is a general vSphere upgrade, which includes upgrading vCenter Server and ESXi hosts.

VMware supports in-place upgrades on 64-bit systems from vCenter Server 4.x, vCenter Server 5.0.x, vCenter Server 5.1.x, and vCenter Server 5.5 to vCenter Server 6.0 and later. The vCenter Server upgrade includes a database schema upgrade and an upgrade of the vCenter Server. Instead of performing an in-place upgrade to vCenter Server, you can use a different machine for the upgrade. For detailed instructions and various upgrade options, see the vSphere Upgrade documentation.