There is an option in VMware where after you have performed an major upgrade of ESXi you can roll back to your previous version. The benefit of this is that you would not need to reinstall your ESXi and its configuration if you had issues with the new software. I had to do this on one occassion in my lab where I upgraded from 6.5 to 6.7 and my VMs would not run because the CPU was not supported in 6.7. Please remember if you are using ISO method to upgrade ESXi please ensure you select "Upgrade ESXi, preserve VMFS datastore". Selecting "Install ESXi, preserve VMFS datastore" does not mean preserving datastore means retaining ESXi as it will still do a clean install of ESXi. This method does not work for vSphere 7.0 as there are changes to the partitions on the boot device.
Below are the steps to roll back to a previous version which is quite straight forward. As always perform an backup of your host configuration before you upgrade or rollback (KB2042141). I have tested roll back where I have performed the upgrade via update manager, CLI upgrade and via ISO.
How do you access the recovery menu?
You may have noticed this screen when you boot up your ESXi host
On your keyboard press "Shift + R" which will take you to the recovery menu before the automatic boot counter times out.
As you can see on this recovery menu we have nothing to roll back as I have not updated the host as yet.
Now go through your chosen method of upgrade for your host and then next time at boot up menu go into the recovery menu (Shift + R) you should see a screen similar like below
You can see the default hypervisor is set as 6.7 and the available ESXi version we can roll back to is 6.5. So press Y" to confirm you wish to roll back and remember it will not prompt again to confirm so make sure that you wish to roll back.
ESXi will reboot and you will be back to your previous installed version.
Its a nice feature for a quick rollback if you had issues with upgrades but don't leave it too long after an upgrade for rollbacks.