After upgrading vCenter, I now had to upgrade my single ESXi Lab host. I mounted the ISO using the BMC out of band utility and booted off the ISO. I ran into an issue with an NSX VIB however and it blocked the upgrade:

Alright, rebooted into ESXi 6.7 and put the host into maintenance mode to remove the offending VIB as follows:
esxcli software vib list
Now spot your offending VIB in the list – mine has the word nsx in it. Then remove it.
esxcli software vib remove -n esx-nsxv
Removal Result
Message: Operation finished successfully.
Reboot Required: false
VIBs Installed:
VIBs Removed: VMware_bootbank_esx-nsxv_6.7.0-0.0.8590012
VIBs Skipped:
