For almost every update, you need to reboot the host. So if that host is hosting your vCenter appliance you should move that appliance to the other host. If that is not possible you could update the host as if it was a stand-alone host.
Thank you, how do you update the host as if it was a stand-alone host ?
You could use the esxcli command line tool: VMware Knowledge Base Or you could use PowerCLI. I wrote a script (Upgrade-Hosts-6.5 - Pastebin.com) that upgrades 6.5 hosts in a cluster when vCenter is already on 6.7 (then you cannot perform a host upgrade to 6.7 using VUM). Anyway, the script copies the zip file with the upgrade to a datastore, puts the host in maintenance mode, and performs the upgrade. You can use the same method using a zip file with the patches.
Remember that the script is made for my environment so needs adjusting for yours... But the goal is to give you an idea of how to do this with PowerCLI. Search for "patch standalone esxi host" and you will get several useful hits...