After you upgrade vCenter Server, upgrade your ESXi hosts. You can upgrade ESXi 6.5 and 6.7 hosts directly to ESXi 7.0.
To upgrade hosts, you can use the tools and methods that are described in Overview of the ESXi Host Upgrade Process.
Caution: If you upgrade hosts managed by vCenter Server, you must upgrade vCenter Server before you upgrade the ESXi hosts. If you do not upgrade your environment in the correct order, you can lose data and lose access to servers.