After you upgrade vCenter Server and vSphere Update Manager, upgrade VMware ESXi hosts. You can upgrade ESXi 5.5.x and ESXi 6.0.x hosts directly to ESXi 6.5.
To upgrade hosts, you can use the tools and methods that are described in Overview of the ESXi Host Upgrade Process.
If you upgrade hosts managed by vCenter Server, you must upgrade to vCenter Server before you upgrade ESXi. If you do not upgrade in the correct order, you can lose data and lose access to servers.