You can easily update your vCenter Server Appliance (VCSA) offline by downloading a product patch ISO and via the CLI.
Download the latest patch from VMware Patch Download Center. Select VC from the Search by Product drop-down menu, then select the correct version (i.e. 6.0 or 6.5). Download the patch ISO and attach it to the VCSA.
Log in to the shell (using SSH or VMRC) and initiate the update:
software-packages stage --iso --acceptEulas
software-packages list --staged
software-packages install --staged
After the update has finished, you will have to reboot the VCSA.