Honestly, I don't understand the question. Why should you have problems with that? vCenter is running on host A or B is irrelevant.

If you are applying updates on host A where vCenter is stored and you need to reboot the host, move the vCenter to another host.

@LucianoPatrão is spot on just move the vcenter to a different host

You unfortunately can't do a VUM update on the host while vCenter is running on that host.
Since vSphere Essentials dosen't support vMotion, your options are actually limited. Some of them are to either update the host using an ISO image (if this contains the patches you need), or to patch the host from the command line using esxcli commands.

André

Yes, @a_p_, only Essentials Plus version has the vMotion feature.

So unless the user has Plus version, it will be more difficult but not impossible.

The only way I see it, is to power off vCenter and move the VM to another host(that need to be done manually in the ESXi console)