This first task to perform during the vSAN 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.
The details and level of support for an upgrade to ESXi 7.0 depend on the host to be upgraded and the upgrade method that you use. Verify that the upgrade path from your current version of ESXi to the version to which you are upgrading, is supported. For more information, see the VMware Product Interoperability Matrices at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php.
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.