If you don't have a vcenter then you can skip the vcenter part of the documentation and upgrade ESXi. They say you have to upgrade the vcenter first because if you upgrade the ESXi hosts to higher version the hosts will not be managed by vcenter which is f a lower version. this disrupts the environment. But since you have just a host(s) and no vcenter you can upgrade it as you usually do.
most of the times nothing happens to the Vms during the upgarde if you choose to preserve the VMFS volume but its always a better thing to keep them backed up.