Starting with NSX-T version 3.1 it's a feature to have NSX-T create it's own service account when registering a vCenter Server as a Compute Manager. In the image below you can see the option enabled in the vCenter connection.

Normally updating a vCenter server appliance is as simple as accessing the VAMI interface on port :5480 and have the appliance download it's own update form the default update URL. And when that's not possible you can always just download the ISO-file and mount that on your vCenter server appliance.