Hello.
If you perform the update starting from DVD or UDB you don't need the maintenance mode.
The maintenance mode is used when installing patches or updating online from the ESXi.
If you have VMs on the ESXi host, the best way to do this is to move them to another ESXi and then after the update move them back to the original ESXi.
It is always advisable to have a backup of the VMs.
If you only have one ESXi with VMs, the option is always Upgrade ESXi, preserve VMFS datastore, with this option all configuration and data is preserved and only the ESXi is updated.
If you reinstall the ESXi you should choose Install ESXi, preserve VMFS datastore, this option preserves the data (VMs) but not the ESXi configuration which you should register the VMs and configure el ESXi (be careful with this option).
You have to verify that your host hardware and current version of ESXi supports the new version 6.7 and you have the respective licenses. If you have 6.0 or 6.5 these are the same licenses key (you don't need anything else)